I've update these devices to 7.1rc to test:

• hAP 2nD
• hAP ac2
• wAP ac LTE

Even after uninstalling some packages (for more than 4MB free space) all of them reported:
Solved it by netinstalling, then loaded the backup and everything was fine.

Then wanted to update my mAP Lite - same issue. And this time it really was an issue... For what ever reason this device does not netinstall.

My guess: The issue is not the free space on its own, but some other update restriction. So I searched changelog for something suspicious and found for 6.42.7:

*) package - do not allow to install out of bundle package if it already exists within bundle;

So I tried with a downgrade in first step:

• 6.49 (split) -> 6.42 (split)
• 6.42 (split) -> 6.48.4 (bundle)
• 6.48.4 (bundle) -> 7.1rc5

That was successful... (Though I did not check any versions in between.)

So is there a general issue upgrading from v6 split packages to v7 (which is available as bundle only)?
Let's hope I will not have to netinstall every single device for the upgrade...

Well it is interesting that it could be fixed by installing the bundle package, I will try that on the ac2.
However, I also upgraded my RB2011 and my RB4011 which had split packages and this worked without problem, so it cannot be caused only by having split packages.
It looked more like a problem specific to 16M flash usage, maybe the space for v6 split packages is freed too late to allow installation of v7.

Perhaps I do not have to netinstall my CCR then.
All other devices have 16MB flash only with split packages installed.

Anyway, would be nice if Mikrotik developers could have a look and fix this.

Ok as a first test I tried to downgrade my hAP ac2 from 6.49 (split) to 6.48.4 (bundle) and that failed with "not enough space for upgrade" too.
So it could be that the lower version is really required. That is something to try tomorrow. [ edit: well, I did not realize that my hAP ac2 cannot use 6.42 ]
Of course 7.1rc5 is not really a bundle package, it is just one very large package with functions that used to be in separate packages...
I remember that when upgrading from 6.49 to 7.1rc5 I saw that one package was installed correctly, then the space error was triggered, possibly because it had upgraded the routeros package and was then left with the split packages installed as well.