The transition period would be painful. Seems like the way to go would be to mandate the end of 100LL, but make it perhaps five years in advance, or more. Could mandate that unleaded conversions be done for continued airworthiness sometime in advance of that date if that would help things along.

I guess I'm assuming that an engine that's been converted for unleaded will still work on 100LL. Obviously that's the case for yours, but I suppose it may not be the case for all. If so, that would complicate things severely.