Last year I created a throwaway Sigil incrementation plugin that might do the trick. Since I only used it for 2 books, you might want to thoroughly test it with a backup copy first.
Note that since all Sigil plugins require Python, you'll need to install Python 2.7. I'd recommend installing
ActivePython 2.7.8.1. (Do not install the 3.4.1.0 version; it's incompatible with this plugin.)
To install the plugin do the following:
0. Install Python 2.7.
1. Select Plugins > Manage Plugins.
2. Click the Auto button after Python 2.7. (You should see a file path.)
3. Click Add Plugin, select the attached .zip file, click Open and OK.
You can select the plugin via Plugins > Edit > AddIDs > Start.
To process all
a tags in the epub simply click OK.
To process only
a tags with a specific class, enter class in the Attribute box and the name of the class in the Value box.