Anything before 12.7 should be upgraded to 12.7, then it is safe to upgrade to current patch, in most cases.
Recommend Back up database before beginning anything
Start by checking database and fixing any errors. If you get a lot of errors, it might not upgrade, but at least try it. SQL has many built in self-healing features.
Sometimes, on an especially old, large database running on a spinner, things can take hours. Try to install a SSD drive (just clone the old system to it) – this can save a ton of time, probably more than the cost of a $20 SSD. Knowing how to do this can save tons of time at client site, plus now you have a backup in hand. I buy 120GB SSD’s in bulk for about $16, brand names too. Keep several on hand a great way to provide value, and increase revenue. You don’t need anything bigger on any server unless it’s a large system. Much easier to deal with than larger drives, and cheaper.
Tools and tricks:
We download every patch and upgrade along with release notes and save them on our server. For a USB drive, I would recommend having a good 32 bit and 64 bit copy of CRE Version 12. I keep several patch levels 12.2, 12.4 and of course 12.71.  save that in directory Old Versions
Because thumb drives are cheap and big, just copy the entire patch and installation directory along with release notes (they can save your ass by reading them for the patch you are installing). Better to have everything than nothing.
It is a good idea to have at least one available PCA license (we ended up with several some years back from a deal that went south use them for demos) If you ever want to move a live database over the wire, you will be happy you have a fully activated copy to accept it and you also need an activated copy to test pinpads and stuff.
I also keep several Acronis images of terminals we sell and service. My rule is if we have to spend more than 15 minutes on a machines operating system, we reimage it, even if it is a server. You can’t allow crap like that to waste tech time and run a client bill up, it’s bad business. Nice thing about Windows 10 is it is much more forgiving than earlier versions. As long as you have the same memory (3 channel or 4 channel) and chipset, Windows will fix itself and work. So if you have older j1900’s out there (don’t we all) just about any image made on one will work on another j1900 and so one with I3 or I5 of the same generation. Our new terminals now use Gen 7 I3 and I5, most of them use older gen 4 or 5. So we have images created for our new super terminal (it rocks dual Ethernet ports you can bridge and plug a pipad into with 1 wire, no unreliable switches.)
Mainly, be prepared. Keep an extra terminal in the car or truck. Keep a couple of switches (yes I know they are bad and ad a failure point) you never know when you need a port. I buy routers in bulk and always have several. Besides making a sale, it’s a lot easier to program a router (rooter) that you know rather than try to figure  out an old one on the spot. (we also sell full on business networks as well). Patch cables. I cannot over emphasize this. I solve more problems by having a bin, organized with about 10 of each: .5 foot, 1 foot, 2 foot, 3 foot, 5 foot, 7 foot, 10 foot, 14 foot cat 6 patch cables. I get the shorter ones without boot, they are relatively cheap on Monoprice.com and a good network tester. You would be surprised how many issues are solved with simple cable. We make bigger ones or keep a couple extra long ones on hand too. On any install we use all new cables, I toss old cables because I refuse to guarantee a system if it isn’t on my material. Use cat 6 cause cables are heavier and you future proof you work, they last longer. I generally quadruple my money on cables. On new installs we have a cables and connector charge ($85 for up to 2 terminals and slides as you ad more stations). Think about your last oil change, there was a shop materials charge of like $3, same idea. It pays for my cable expense and makes sure customers have good quality cable for system. We also have a PIT charge.

That’s all I can think of now.