Blaise has always sought backward-compatibility. This means that a system developed by Blaise 4.8 can, in principle, also run in Blaise 5. However, the reverse may not always be true. It is recommended to check if older applications still run correctly after the transition to a new version.
Changes from previous versions are described in the Blaise 5 help file under "What's new". Also look at the chapter "Blaise 5 for 4 developers". You can find help through the Control Centre in Blaise 5 or online via this link.