How can I change my youth from one championship to another

Your junior team follows your professional team.
If you change leagues, it changes to join the same one.

Personally, I've left the field. Rest of Europe but my young team is still in that league!

So don't dwell too much on this 'detail'!

It can take a season for a junior team to 'graduate' from the old league.

Left at the end of S.135... to be seen in the pro season... but I don't really care, it's just a detail