I would say this depends on the school, how good their record-keeping is, and how standardised their course is.
I'm lucky in that both of the schools I've used for training were excellent in this respect. I changed PPL instructor towards the end of my course, and it didn't delay me at all. Likewise, when my regular PPL instructor was on holiday or off sick, I would simply fly with someone else who would read my student notes left by my regular instructor, and we'd carry on where I'd left off.
Apart from my PPL training, the other course where I switched instructors was a 10-hour aerobatics course, where I don't believe I flew with the same instructor for any two consecutive lessons. But every single time I flew, the instructor knew what I'd done previously, and what was next on the agenda. This came down to the fact that the school standardised the course sufficiently that each instructor was confident that I'd learnt the previous lessons well enough to be able to progress onto the next one without the need for re-capping.
Don't worry about it - and good luck!
FFF
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