Yes, I think you are right that a number of seemingly small boundary changes can ultimately re-colour the therapeutic landscape. Even when these changes are explicit, negotiated and beneficial, if the warning bell goes off we would be foolish to ignore it - especially if there are previous life experiences of boundary violations which have taught us to be wary of shifting boundaries.

I suppose it makes sense that your boundary changes with previous therapists don't necessarily mean that changes with your current therapist will be ok. After all, the boundary is a mutual meeting and not just characterised by your self.