Let me just start out by saying you really need to be keeping your Exchange Servers current. It’s not just about support, but the additional fixes and, most importantly, security updates. (And If you are running in hybrid mode, it’s an absolute requirement to be at the latest or previous version.)
So, do yourself a favor and upgrade as soon as possible after a new release and you won’t run into the dreaded disappearing CU problem .
So what to do if that “bridge” CU is missing? Well, for the last couple of years, the advice is to call Microsoft Support, get the CUs that are no longer available publicly and step your way through the CUs and .net upgrades.
I would still recommend that option for the most part. It’s the tested and proven way to upgrade.
There is another recent choice however that you may have missed. Upgrading directly to the latest supported .Net version, then the latest CU. I’m not making this up!
Check it out:
When upgrading Exchange from an unsupported CU to the current CU and no intermediate CUs are available, you should upgrade to the latest version of .NET that’s supported by Exchange first and then immediately upgrade to the current CU. This method doesn’t replace the need to keep your Exchange servers up to date and on the latest, supported, CU.
Microsoft makes no claim that an upgrade failure will not occur using this method, which may result in the need to contact Microsoft Support Services.
You are on 2013 CU9. You want to upgrade to CU19. I would recommend upgrading to CU15, then .net 4.6.2 – then CU19, then .net 4.7.1 following this article.
However: If that isn’t possible for some reason, you could go straight to .net 4.7.1, then install CU19.
Note that caveat again: Microsoft makes no claim that an upgrade failure will not occur using this method, which may result in the need to contact Microsoft Support Services.
What that is saying is that Microsoft will assist you if this fails, however it’s not a tested scenario.
So bottom line: Keep your Exchange Servers upgraded and happy in a timely manner. But if that is not possible for some reason, you do have another option- one that Microsoft support will now assist you with in the event it goes sour.