Stable builds are generally released the same day as official Blender builds. You get free updated builds every week too to always get the latest improvements made in master if you prefer having access to bleeding edge features.
You can sidegrade to any version of E-Cycles any time by only paying the difference between the version you own and the new version of your choice. Just contact me and I'll send you a coupon.
Yes, E-Cycles is 100% compatible with the official version, including assets, materials, add-ons, textures, etc. everything works out-of-the-box. You can start working immediately.
If you buy the new version before the new year, you get it half-price. You can also upgrade later and get 34% off which gives currently access to E-Cycles 2021 for $99 instead of $149 and E-Cycles RTX 2021 for $199 instead of $299.
The coupon code for that is EC-SecondYear. Please be sure to be logged-in with the account containing your previous E-Cycles version for the coupon to work.