The 2.5 will be rolled out in two stages:
Stage 1: Openbeta 1.5 will be updated to OpenBeta 2.5. If OpenAlpha is installed the updater will pick up the installed modules from it and will suggest to remove it after the install. As usual, common files will be used from 2.2: if you selected to remove 2.2, the files will be moved. If you selected to keep 2.2, the files will be copied (actually, hardlinked).
Stage 2: Release 1.5 will be updated to Release 2.5. Before the update the updater will suggest to keep 1.5 as a separate install. If you agree, it will download the special DCS 1.5 installer which will move the files (and download what's needed if any) from your existing 1.5 install to a new location of your choice. After that, if there's OpenAlpha 2.2 is present it will do the same as Stage 1. If no OpenAlpha found, but there's OpenBeta 2.5, the update will pick up the list of modules from OpenBeta 2.5.
At the same time as Stage 2, those who only have OpenAlpha 2.2 installed will be prompted to automatically download and install Release 2.5 which will, as noted earlier, ask to remove 2.2.
This is as detailed as it gets.