Late last year, we revisited a pension plan whose 2008 retirees were not offered the 75%J&S option. We offered them the option to elect the 75% and the difference from original election would be reconciled. A participant originally elected to not withdraw employee contributions and take the 50%. Now participant would like to elect 75% and withdraw ee contributions. Would there be an issue with this?

Could be. Depends on your plan amendment.
But (likely) the plan amendment is not done, so it depends on what the sponsor wants to do. On a non-discrminatory basis of course.