If I have an Aegis ReVi15 with a beneficial side effect (warning shriek), can I keep that side effect if I develop a higher level version later, or is it lost then?
And a level 15 aegis is not possible.
Rituals are at least level 20.
Heh, "no" is not a perfectly clear answer to an "either/or" question.
No, the side effect is lost - sorry.
You'd have to account for it via the Spell Guidelines in the design of the new spell (CrIm), or try for some sort of breakthrough (See HOH:TL, p 26 etc).
And you could have a Level 15 Aegis, but the minimum Ritual Level is 20 - see p 114, top Col iii, and study some Arts ya slacker!