Hi,
There will be many issues in directly creating packages and deploying them to aem 6.3-
1. There could be content nodes which might have names with special characters those will be removed while installing , you might loose huge amount of content as in previous version those node names was not a issue.
2. On installing the dam package huge number of workflows will get triggered which will cause high load on servers.
3. Users and groups permissions might be difficult to package but in place upgrade will solve these issues.
4. Installing packages will take huge amount of time, in place upgrade will be too fast.
5. We migrated from aem 5.6 to aem 6.2 we tried creating packages and installing it over we faced many issues.
6. Anyways pom & code needs to be updated in any case
Hope this helps.