Hello,
Thanks for writing. You can have any number of different versions of our controls on the same machine, installed in the GAC (in fact, this is one of the main reasons we use the GAC).
The problem you have is most probably related to the fact the Visual Studio.NET silently adds a new line in web.config for each assembly you reference. However, when you remove the reference, it does not remove the line from web.config and still searches for that assembly, hence the problem you are having when removing 6.3 and then adding 7.2
Could you please open your web.config file and search for any references to Infragistics 6.3 assemblies, and if there are some, just delete them? Also, you may try running the "6.3" search for the whole solution to see if you have them referenced somewhere explicitly in ASPX files - you will also need to remove them from there in that case.
Please, let me know if this helps.