md5 was considered broken
This is a wrong statement, MD5 wasn't ever broken nor is. The only way it would be considered as so if you or someone can actually reverse it and that, so far, is impossible.
Being open to Brutte-forcing doesn't make anything "broken" as no known algorithm is resistant to it. All you can make is it slower to bf, not prevent it.
MD5 is just "fastest to brutte-forced than others", along with those "rainbow tables" (which is just a database with pre-computed hashes, btw).
Sorry but cryptography defines broken very strictly. A hash function is considered broken if it is possible to generate a collision in less time than brute force. It might still be hard or difficult for the average person but since it's possible, it is defined as broken. If it's possible to solve the hash function easily (by cryptographic standards) then it's considered completely broken.
For MD5, it has been demonstrated by researchers in 2004 how to create different files with the same md5 hash. In 2008, a successful attack was made on SSL and in 2009, an attack was published that is allegedly doable within minutes on a normal computer. MD5 is considered completely broken cryptographically.
About that "old/new", it refers to: old -> Jed time / new -> some time after M'Tux bough that, not as "old before the attack/new after", as I believe you were assuming.
You believed wrongly again. Let me put it into a timeline so it's clear what I was referring to
1. Original weak hash passwords with plain unsalted md5
2. mtgox realizes it's bad, changes hash to a slightly better but cryptographically just as weak BSD md5crypt
3. you signed up for an account or changed your password so your data in the database along with many others are using the salted md5crypt
4. hacker obtains database
5. easily cracks passwords from #1 and from released files, cracked the newer hashes from #2 too.
If #2 was a change to a more secured hash such as SHA256, then #4 would had only practically only affect the old/idle accounts from #1.
If mtgox did something between 2 and 4 to notify/force users to change their password, then the loss to #4 would had been further reduced, but only if a more secure hash was used.