In Whidbey, there are four commands that involve converting spaces to tabs and vice versa

Edit – Advanced – Tabify Selected Lines – Replaces the leading whitespace on a line that contains the selection with tabs.  Doesn’t matter whether the leading whitespace is selected, as long as part of the line is selected.

Edit – Advanced – Untabify Selected Lines  – replaces the leading whitespace on a line that contains the selection with spaces.  Doesn’t matter whether the leading whitespace is selected, as long as part of the line is selected.

Edit.ConvertTabsToSpaces – converts selected whitespace to spaces.  Note: this command is not available on the Edit menu, so you’ll either have to bind it to a keyboard shortcut, put it on the menu yourself, or run it from the Ctrl+D thingy.

Edit.ConvertSpacesToTabs – converts selected whitespace to tabs.  Note: this command is not available on the Edit menu, so you’ll either have to bind it to a keyboard shortcut, put it on the menu yourself, or run it from the Ctrl+D thingy.

Notes for VB:
The Edit – Advanced – (Un)Tabify Selected Lines isn’t supported. 

You may have to uncheck Tools – Options – Text Editor – Basic – VB Specific – Pretty Listing (reformatting) of code to use Edit.ConvertTabsToSpaces and vice versa – provided you’re using tabs as the default and want to convert something to spaces.  But the other way will work fine (if default is tabs and you’re converting spaces to tabs).

Happy Visual Studio’ing!