There is a new Version Utility in 2010.2 that you can use to make this switch.  Once you pass it your source (solution, product, website), it'll analyze and recommend upgrading to 2010.2 3.5, but you can select the Upgrade Options and change it to upgrade to CLR4.0, and then select 2010.2 from the drop-down.

Here is what worked for me...

I did a solution-wide find/replace operation that searches for "Infragistics35" and replaced it with "Infragistics4".

A couple caveats...

• Better make a backup first
• Gotta include .aspx pages to make sure you update any tagprefix declarations
• Gotta include the web.config
• Gotta make sure any dll's you put in the bin folder are the 4.0 version and not the 3.5 version
• Be aware that putting the assemblies in the GAC on the server is a whole new process (one I haven't figured out yet).  Can't use c:\windows\assembly to do it with the 4.0 version.  I've been forced into using the bin folder instead of the GAC until I figure it out.
• Once done and everything's working, convert your toolbox items from the 3.5 version to the 4.0 version.  Delete all Infragistics tab sections, and then use the "Create Toolbox Tab for CLR 4.0" utility.

I've gone through all of this, and everything works.

One nagging little bug remains...  Every once in a while, Infragistics will automatically place a reference to the 3.5 WebDataGrid assembly in the web.config, resulting in the site not compiling due to ambiguity.  At least I know what to look for now, and just remove the reference whenever it appears.