OK ...... Well , what I would say is the Tensioner was probably overextended before as I had mentioned , you just did not know it , it was probably on the verge of coming apart , then when you reassembled , it was only extended to where the detents allowed , because as you mentioned , you did not alter the height , if anything you increased it by installing the new head gaskets , but that amount would be insignificant and not be enough to affect the chain's slack enough to physically make a difference
Either way , the chain is stretched and needs replaced .... I personally would be concerned that the tensioner now has some side to side slop and will fail at some point because of overextending or binding as it must have .... so I would still suggest replacing both , if anything as a precaution to a future failure that could cause extensive damage if it does fail of valves hitting piston , etc. , (better safe than sorry as they say)
My guess is the chain did not suddenly stretch , it was already in the state it is now , but disassembling is what changed the tensioners position . thus creating the slack , again , I am suggesting the tensioner was jammed and holding the tension to some extent , now that it is not , there is a large amount of slack , and it will not take up the gap until it gets jammed again , and possibly causing real damage .....
