My situation is almost identical to this question. I have just bought a used laptop that has Windows 10 Enterprise LTSB v1507, Build 10240. Windows is activated, but i don't know the product key. I want to get the latest update on it (Spring Udate 1803) and using the answer to beforementioned question i downloaded iso of Win10 1803 to do inplace installation, but i can't find any info regarding activation status. Will i keep the Windows activation when i install Enterprise 1803 from iso or will it ask for product key (which i don't have)? I've googled that after Threshold 2 update activation info is stored in Microsoft database and you no longer need to provide a key when reinstalling, but Threshold 2 was 1511 update and my version is 1507 and LTSB has no updates available.
So how do i upgrade my Enterprise LTSB to something new and keep my Windows license intact?