Implants that have been in place for several years will have stretched out the soft tissues, and will have created a pocket for the implant already. The initial pain and discomfort should be minimal. However, the other factors involved are if the implants are ruptured (silicone) and possibly making removal more difficult, or if there is a decision to go to a smaller implant and having to address extra skin or creating a smaller pocket size for the implant. Recovery can be quick on the order of 3-5 days for simple removal and replacement, to 7-10 days if drains have to be placed, or if a site change to submuscular position or breast lift is necessary. In either case, physical/aerobic activities are usually limited for a minimum of 3 weeks, and certainly weightlifting/yoga/chest exercises limited for 6 weeks.
Hope that helps,