Depends where you're flying. Different countries have different rules.
In Oz everyone changes the subscale passing the transition altitude/level, CTA & OCTA. At least everyone is on the same setting if they're in the same area.
In the UK it can be changed as soon as you're cleared to climb or descend to above or below the TA/TL unless you've been asked to report passing a altitude or level on the way up or down, in which case you have to leave the change until after the report. It's even more complicated because the UK uses a myriad of TA/TLs.
Guess which system is simpler....