I would look thru all the help docs as mentioned above.But a few personal suggestions from long, although limited upgrade experience (i.e. you do it for each version maybe, but don't repeat):
- If your current DBMS in place is compatible with 10.2./10.3 (remember to test on a copy first)
- if you have space and admin permissions on the machine, you can create a new GDB and copy/paste in ArcCatalog. If you have topology in place, you may need to validate the full extent (and may have to remark exceptions. This happens if you "replicate", but I don't remember about copy/paste).
- or backup and restore to the new machine. You can not restore to the same machine because restoring uses the same GDB name (i.e. conflict)
- The actual "upgrade" for the GDB is usually done in ArcCatalog, and at least for more recent versions, it is usually just a right-click->Properties and it has an "upgrade" option (or will tell you it is the same version). It will upgrade to the version of your ArcCatalog....but again, assuming you have a compatible version of you DBMS.
- fyi, we've had SQL2008 for many years/versions of SDE, but with 10.5.x, SQL 2008 is not supported so we are upgrading to SQL2014. Although SQL2016 is available, not all of our workflow/software is compatible with 2016 yet, so keep that in mind if in SQL.
- I don't remember if 9.3.x still allowed for "services" (ie port 5151, etc), but moving forward you use direct connects for all. Even if you are already using direct connects, you may still need/want to recreate them (beside the fact that you need to point to the new gdb)
- Newer versions of SDE/enterprise are much easier to manage with ArcCatalog, but if you have some .bat files using the old command line, some of the old commands will still work but may need to be modified if they look at 5151, etc.
As mentioned, we are in the process of upgrade our SDE/enterprize from 10.2.x to 10.5.x. Not nearly the leap you are doing. I think jumping from 9.x to 10.x was the most worrisome, but all went smooth as I recall. We actually were not able to make the leap into SDE from coverages for most of our master databases due to 9.x lacking the topology rules we needed, but since then, I have been very happy with our setup (with a few format tweaks along the way). Needless to say, a lot of work went into getting SDE to work with our vector data the way we wanted.
Prior to 10.x, we mainly used SDE for storing our raster data. Back then, that was the best option. Now, the best option for us is using FGDB and mosaic datasets. If you have raster in your SDE, you may want to look at that as an alternative.
