If you leave AD one day and swear in the next at the Guard, there is no break in service. If you stay in IRR ( or navy equivalent) and come to guard, still no break in service. Both of these have pros and cons. You must work well ahead of time to pull off leaving AD one day and coming into guard the next. It can be done and is the preferable method because of complications of going to IRR to another reserve status. I have had 3 navy and 3 marine pilots do both ways. Depends on your situation. Break in service = bad. You will probably lose a pay grade, hapened to me.