I think foobar2000 can do this with its Masstagger. In some versions the masstagger came included by default, in other versions it needs to be installed as a plug-in.In the version I'm using (1.1), with masstagger installed, I can right-click a group of files in the playlist, select "Tagging", and then "Manage scripts".If I understand you right, the action you want to perform is "Copy values between fields", for example copying the RELEASETIME field to the TDRL field.You might end up with both fields afterwards, but you can simply delete the one you don't need anymore.
I don't think there's a way to do the conversion and tagging in a single step.
My experience with foobar has been that it copies over all the tags during conversion, does it not do this? I mostly stick to the default tag fields (Artist, Album, Date, etc.) so it may be that I just haven't noticed.