There are some problems - my PC is basically an elven Fighter/Magic-User but is well outside the racial level limits. But some high-level spells are an integral part of what's gone on so far. Magic items are another issue; I'm not sure how we'd convert a lot of them.
Anyway. Has anyone ever done this before? Any ideas?
One of the first things I did with 3e was sit down and try to convert some of my past PC's into the new system.
It didn't really work. I had a high level Elven Thief/M-U roughly equal to 13th level. Core there was just no way. I ended up eventually trying some sort of Ranger1/Rogue4/Wizard5/Arcane Trickster3 or something like that but even then it just doesn't work out. If the character was a wizardly as my 1e PC, then he couldn't be as thiefly. If my character was a thiefly, he couldn't be as wizardly. To actually get the feel required making him about 16th level in 3e.
Your going to have a similar problem going back the other way. Your 20th level character is probably in 1e terms about 28th level. If you try back porting though, the rules are going to say that that's impossible either way. With some generousity from the Unearthed Arcana and some racial subtype min-maxing, your character might be a 10th level Fighter/12th level M-U at the very upper limit of his power and no future advancement possible and roughly the equivalent of a 14th or 15th level 3e character.
Even ignoring racial limitations won't solve your problems. Levels won't work out exactly. What 20th level means in 3rd edition with a PrC probably involved is different than what it means in 1e. Plus you are going to find that 20th level is WAAAAAYYYYYY beyond the range of what 1e by default (especially post UA) expects to be able to need to challenge. Remember that 1e monsters are rated by 'challenge' according to the expected depth you'd find them in a dungeon 1-10, where 1 is the first level of the dungeon and 10 is the tenth level. Recall also that each level of the dungeon corresponds roughly to player level, in that the second level of a dungeon is basically a good challenge for 2nd level characters. Well, much past 10th level, and nothing in 1e is a real challenge without significant DM metagaming, invention, or extrapolation. There are only a handful of official 1e monsters that can challenge characters much above name level - classed NPCs, greater Slaad, 16 headed X-Hydras, Arcanadaemons, Lich, Shambling Mounds boosted by lighting, Jann lords if you ignore the typos and treat that HD as real, Korreds if you abuse the mechanic of the laughter power, classed Vampires, Thessalhydras, near top age category dragons, etc. At one point I could give you a full list, but purusing the 2e adventure 'Axe of the Dwarven Lords' is more or less a greatest hits in that regards. By and large, if you're 12th or 13th level, nothing stock is a major challenge and DMs are going to have to rely heavily on their ability to smith monsters capable of challenging you. Also note that a significant portion of that list is things with high level spell ability. It's not necessarily easier to run high level 1e play than it is high level 3e play.
Magic items should straight convert by name or spell-like ability. That shouldn't be much of a problem, but you will again end up probably wildly out of expectation on power level.
Figuring out your attributes are going to be a bigger issue. 25 int in 1e doesn't mean what it does in 3e and vica versa.