As explained in bug 138398 comment 22, you need to explain in detail, what specifically you do, and provide some texting samples. The screenshot does not allow to see what "I switch database source" specifically means, i.e., which commands / functions you use.

I *suspect* that you might expect that the highlighted database in the Data Sources panel (Ctrl+Shift+F4) means the data source used for the mail merge document - but it is not so: the bold font on the data source in the panel only shows which table is displayed on the right. The data used for the mail merge is obtained from the data source set when you created the fields (the mail merge field properties would point to the original database, unless you change that manually).

Hi,
Thanks for your reply Mike.

When I selected the new database in the right panel, I thought that the database source changed for the fields. So it doesn't.
OK.

But there's a problem when I change database source.

- The 2 databases (based on calc spreadsheet with same sheet names) have the same construction/fields : only the records change (different values and different number of records).
- So when I change the database source via menu "Edit" -> "Exchange Database...", all works fine : the fields are updated. Manual changes are not necessary.
-I save the document.
- When I re-open the document, all seems OK (fields names with right database source, right database highlighted in the right panel) BUT I've the old database displayed at the top.

So it's not a specific problem with email mailmerge but more general. It seems that when I save the modifications, the database source is still the first attached, not the last.

(In reply to Thierry Munoz from comment #2)
> BUT I've the old database displayed at the top.

What is "the top"? Please always be as specific as possible. You can point to specific place using verbose textual description (like "title bar of the text document", or "left panel of Data Sources window (Ctrl+Shift+F4) shown above the text document", r whatever you refer to - I really don't understand), or you may provide screenshots with arrows pointing to something - but make sure others, who do not look over your shoulder, see what you mean :-)

Created attachment 180182 [details]
Screen description

The screen displayed when I open the saved document.

Rem : I'm not a developer, not familiarized with the usual technical terms and english is not my native language (writing these messages take me a lot of time). I'm simply trying to contribute at my low level.
So I would quite help and make your task as easiest as possible but your speakers have not obligatorily your skill...

(In reply to Thierry Munoz from comment #4)
> Created attachment 180182 [details]
> Screen description
> 
> The screen displayed when I open the saved document.

Is this with version 7.2 / 7.3? I cannot reproduce using Version: 7.3.3.2 (x64) / LibreOffice Community
Build ID: d1d0ea68f081ee2800a922cac8f79445e4603348
CPU threads: 12; OS: Windows 10.0 Build 19044; UI render: Skia/Raster; VCL: win
Locale: ru-RU (ru_RU); UI: en-US
Calc: CL Jumbo

(I'm also not a native English speaker. The advise I gave you in comment 3 was intended to help you in the future when you provide information to others - you would know better what others are expecting from you. Answering with "people not necessarily know this and that" helps no one ;))

Problem under LO 7.3.3.2 :

Version: 7.3.3.2 / LibreOffice Community
Build ID: d1d0ea68f081ee2800a922cac8f79445e4603348
CPU threads: 8; OS: Linux 5.4; UI render: default; VCL: gtk3
Locale: fr-FR (fr_FR.UTF-8); UI: en-US
Calc: threaded

Problem under LO 7.1.8.1 :
Version: 7.1.8.1 / LibreOffice Community
Build ID: e1f30c802c3269a1d052614453f260e49458c82c
CPU threads: 8; OS: Linux 5.4; UI render: default; VCL: gtk3
Locale: fr-FR (fr_FR.UTF-8); UI: en-US
Calc: threaded

Unfortunately, I can't repro. Possibly OS-specific; or maybe a sample document with two databases could help to see what's special in it.

Also no repro, tested Win and Lin GTK3. 
This may stay as Needinfo if you manage to make repro steps with some additional detail, or it may be closed. 
Or you should prepare a minimal sample that you can share. You might see that something was leftover, I guess. 
You should try with LO User Profile reset/rename.

While looking at this I notices something that may or may not be the same bug.
I changed a database source Calc1.ods with the similar fields Calc2.ods via menu "Edit" -> "Exchange Database...",.
Then I change back a database source Calc2.ods with Calc1.ods and all looks OK. 
But after I leave LO open with Data Sources and come back, Calc1 is missing from Data Sources list and from "Edit" -> "Exchange Database/ Available databases, it remains in Databases in Use.

Dear Thierry Munoz,

Please read this message in its entirety before proceeding.

Your bug report is being closed as INSUFFICIENTDATA due to inactivity and
a lack of information which is needed in order to accurately
reproduce and confirm the problem. We encourage you to retest
your bug against the latest release. If the issue is still
present in the latest stable release, we need the following
information (please ignore any that you've already provided):

a) Provide details of your system including your operating
   system and the latest version of LibreOffice that you have
   confirmed the bug to be present

b) Provide easy to reproduce steps – the simpler the better

c) Provide any test case(s) which will help us confirm the problem

d) Provide screenshots of the problem if you think it might help

e) Read all comments and provide any requested information

Once all of this is done, please set the bug back to UNCONFIRMED
and we will attempt to reproduce the issue. Please do not:

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b) update the version field in the bug or any of the other details
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