I am /was in the same situation... I've renewed my ITAS once and have now just gotten my ITAP, both times I just made sure to report in time (according to the immigration office here that means between 3 months and 1 week before the expiration date of your current ITAS) to the immigration office. Both times took less than 2 weeks to process and I did not have to leave the country whatsoever to get my ITAS renewed / changed to ITAP. I do not know if it makes a difference, but as said: I am on a spouse ITAS too...