If the "Show whitespace" option is turned on, diff view displays dot symbols for both tabs (four dots) and spaces. If the identation in the file is changed from tab to spaces (and 4 spaces are used for each tab), the user cannot see the whitespace change.

I'd suggest to replace tab with "»" plus (tab size - 1) spaces (3 for tab size 4), so the alignment stays the same.

Example (tab size = 4):

»   line with one tab
»   »   line with two tabs
»   »   »   line with three tabs