Interpolation algorithms (None, Bilinear, Bicubic, Lanczos) require a minimum set of pixels in the vicinity of each pixel to be interpolated without perceptible loss of quality.

I propose that this dialog get a new field like "modify the old resolution only if the new one is less than" : [r].
The default "r" ratio could be 50%.

If you set 1000% for the "r" ratio with a 500 dpi old resolution and 600 dpi new resolution, it will transform the image with a 600 dpi resolution.

If you set 50% for the "r" ratio and a new resolution to 200 dpi :
A. a 500 dpi old resolution will be changed into 200 dpi (because 200/500 < 50%)
B. a 300 dpi old resolution will be unchanged (because 200/300 > 50%)

In the B case, it prevents a loss of quality (because of interpolation algorithms).

In the B case, you can still use this dialog box in order to convert an image from one format to an other (but the resolution will remain the same).