A given body is pretty much fixed, although one could in theory alternate given enough time and effort.
It’ really a question of the demon and their power/strength/ability.
Demons can grow in power over long periods of time, just like Astlanians. Most simply don’t, or not in ways that are measurable, at least not on mortal timescales.
And I should be clear, unless they are really stupid, and some are, demons do “grow in experience’ however some of that doesn’t directly translate in a significant rise in “power” as defined by the colors. i.e. measurable mana or animus levels.
For example, a type II demon might eventually become a much better servant, much more able to anticipate the needs of a master and avoid pain and suffering, and better please new masters sooner, etc. Does this change their power level? Not by a huge amount, except over very very long time periods.
Assume a Type II demon doing this for a thousand years, and then a brand new II. Technically in terms of sheer “power” (whatever that is in the mirror) they are both Fiends, but the thousand year old fiend is going to be much more experienced and this would translate into real world advantages in doing their duties serving their master. But would it make them better in combat? Perhaps/probably, but only due to experience and having been in more fights, and if the thousand year old demon had never fought before? Then no it probably wouldn’t help.
This comes back to what does “more powerful” mean.
There is sheer, measurable power: mana/animus and then there are less measurable quantities of experience, skill, willpower, determination, motivation etc.