Not unprecedented. Al Shepard pretty much picked himself as commander of apollo 13 after a long absence due to illness. Thanfully, it was overulled and he was given Apollo 14.
Has Shepard ever commented publically on dodging that little bullet?
However I guess with all the training etc that it wouldn't of made any differnce if Shepard had flown on 13 and not Lovell?
Well, Shepard died years ago.
And despite training, yeah I think it would have made a big difference between Shepard and Lovell for 13 and 14.
Shepard's only space experience was as first American in space (15 minutes and only sub-orbital).
Lovell had a few Gemini missions under his belt, plus the incredible Apollo 8 mission. He was a veteran, the most experienced astronaut at the time. Couldn't have had a better man in command during the 13 crisis.