Yes, upward merging (from an older Mendix version like 10.15 into a newer one like 10.24) is technically possible, but merge conflicts are very likely.
Conflicts typically occur in:
System modules that were automatically modified during the platform upgrade
Deprecated or changed widgets
Marketplace modules with different versions between branches
Java actions / custom widgets affected by API changes
At this point, there are usually two options:
Resolve the merge conflicts manually
This can work, but it often leads to subtle or hard-to-detect issues later, especially with upgraded system components. For that reason, this approach is generally not recommended.
Open the older branch and manually copy the required changes into the newer branch
This is the safer and preferred approach, as it avoids carrying upgrade-related conflicts into the newer branch and gives better control over what is brought forward.