British citizen/ got Z type WITHIN China/ Can I travel out and back in without Z type entry stamp

I came to China six months ago originally on an L type. During the first 60 days I found a job. They sponsored me and managed to get me a work Z type and work permit without me having to leave the mainland, despite information telling me that foreigners should go to their home country or HK, so I have been here for the remaining four months on a work Z type quite happily without problems. Long story short, I've changed type without leaving. Now, the question is, because I didn't technically re-ENTER PRC and get the entry stamp for my WORK Z type, will the border control note this as a gross overstay on my previous L type or suspicious obtainment of my work Z type? I will be travelling to Japan and back soon and was wondering if I'll be punished for this mismatch of entry stamps? I've contacted the people at my workplace and they've told me there's been a new policy which allows foreigners to switch visas from within the mainland but I'd rather get an outside opinion from people who might know the policies better, as I feel this case is not too common.