A player in my campaign has decided she prefers the 1st edition magic-user to the current 4th edition wizard. So, she asked me if she could change her wizard into a magic user, modified a little to fit into the new rules. I was happy to oblige, for I think magic users have always been the most fun to play, and there isn't really an analog in 4th edition.
Because hit points have different values across the two editions we could not possibly use 11d4 for the hit points of an 11th level character (the expectation of 11d4 is around 22!) Likewise, issues of conversion from AC/saving throws to AC/reflex/fort/will have to be handled. Hence, we decided to simply replace the spells of a wizard with 1st edition spells and award the number of spells known according to the 1st edition charts and intelligence score.
Here are some of the things we have done to make the conversion in spells work across the editions. First, the unit of measurement used to be the inch and this corresponded to 10 feet; because of the new standard where each character takes up a 5 ft square, we have defined an inch to be 1 square in 4 ed. Second, there is the issue of saving throws. In 1st edition, every monster had a chance to make a saving throw versus a spell with success depending on level. In 4th ed. saving throws are (essentially) the same across levels, and saving is too commonly successful for this purpose. Hence, we have followed the example of fourth edition by using spells vs. reflex/will/fort, depending on the nature of the spell.
We may have accidentally created a super character class: a 1st edition magic user with the hit points and defenses of 4th edition, but why not? We are having fun.