There's a "tabify" command in
Edit > Advanced > Tabify Selected Lines
(and the Power Tools 2010 also provide this functionality on a per-file basis) but is there a way to do this for all code files in a solution?
ReSharper has a Clean Up command but the only half-suitable option I found there is to run formatting on all files which does more than I want (I don't want to run a complete formatting, just tabifying).
If you have added the Microsoft Productivity Power tools extension (which if you haven't I would recommned) it adds an option to tabify files. This does not apply across all files in a solution, but it's prompted for when editing each file, on a per file basis. Not quite what you're after but a help.
Also you might try setting your IDE editor settings to use tabs, then do menu-edit-advanced-format document (CTRL+E,D). This will replace groups of tab length spaces with a tab, and that should be scriptable for all files in the solution via a macro.