I known nothing about tuning and key, tbh. However, I am aware that songs can be adjusted to fit the vocals. That is why, I assume that the band will have to do some adjustments ( I am not sure if "downtune" is the right term to call it) so that Jon isn't forced to sing the classics they way they were recorded back in the day. There must be a way to do this without ruining them.
P.S. Joey Tempest of Europe has been singing the back catalogue differently ever since the band reunited in 2003 and the old songs do not sound bad at all, even though Joey's vocals are not that "high" anymore. Those of you who are well versed in this field will tell if a similar strategy can or cannot be applied to Bon Jovi.