vBulletin SHA1 Workaround?

[agallian86]

I've upgraded to vBulletin from SMF, and am looking to not have my users reset their passwords. The problem is that vBulletin uses an md5 hash while SMF uses SHA1.

So, my theory is to, during the login process, write some php code (or something else?) to essentially:
-capture input password
-convert password to md5 and check with db, if true -> login (the typical behavior), else...
-convert password to sha1, check with the db, if true ->...
-save inputted pass as an md5 hash, go back to step 2.

I've read conflicting things about how many md5 calls there are and where they are relegated to. Can anyone point out a specific place to modify (i.e. member.php) and/or give an example of how to accomplish what I need?

Thanks.

P.S. I wrote tech support on vbulletin.com and my ticket assistance said it was possible and to refer to you guys for some code modification help.

I've been informed (and learned via searching) that I need to pay most of my attention to the vbulletion_md5.js, login.php, and functions_login.php files.

I am not an experienced coder (for these languages at least) and could use some help implementing a SHA -> MD5 workaround; the workaround doesn't have to be done the way I've explained above by any means, as long as it's transparent to the user and they can login to their existing SHA'd password.