I wrote a complete tutorial with a small use of JS to solve this issue.
Visit the tutorial
What I did was:
- Set off the parse for XFBML tags in the JS SDK
- Adding a function-call to the JS SDK with a date parameter that you manually set it once with a date that posts published after that date will start using the 'href' version, and posts published until that date will use the migrated version.
- Adding another attribute to the fb:comments tag which is 'data-date' and in it's value you place your post date (dynamic code).
- Calculating the dates in a JS code, and base on the result it's removing the not wanted attributes, and finally then it's render the XFBML tags using FB.XFBML.parse