Can you make a masterkey out of a change-key?

I've seen several systems where the master key was cut deeper, hence allowing extrapolation. May not be the case now though, it certainly throws a lot of extra work in to have the change keys cut the deepest. If this isn't the case, then yeah, need some key blanks. If key blanks can be obtained, duplicate as many as there are pins except with the position for one pin on each left at max height. Then file down each "max height spot" until it opens, combine all of them on a final key blank. If it's possible to see a master and determine that it's narrower then the change keys, just file down one pin position at a time, testing that it opens each time. There are plenty of papers online on this (as well as being covered in LSS+). I've done the "easy" route (i.e. master key lower then change key) twice, but I don't have a master lock system to experiment with so I can't really write up a full instructable.

(this obviously applies to normal pin tumbler locks, not dimpled, twisted or otherwise enhanced stuff (medico, abloy-type, whatnot)

It depends on the master key system-if it's the "rotating constant" method, good luck. Also, if you just want to do this as an intellectual exercise fine, but realize you may be putting the safety of hundreds of people and millions of dollars worth of property in danger. In the 80's I worked full time as a locksmith; one of the supers at a 900 suite highrise lost the master. The proceeds for changing all those locks bought us a new van.

Rotating constant merely convolutes the problem another step by reusing some depths as either master or "normal". That way, it's not instantly obvious which one is "master" (in the older locks, the one that isn't on your key), meaning switching every cut in the normal key to the other cut won't necessarily be the master - it could be you share a cut (or up to all minus one) with the master in order to confuse you. Never the less, some combo of the cuts that open your "normal" will be the master, i.e. at worst 2^pins versions. It can be narrowed further with access to several locks. Algorithmically it isn't terribly secure - 7 pins makes it a combination lock with 128 combinations.

Really, locks count on people being fairly dense at all this. Did swapping all those locks really increase security? Probably not that much. Sure, people demand pouring more money into it, but that's peoples insecurities. Most locks are a complete joke, blocking only the most half assed attempts at breaching them. That, however, is good enough to work. I don't reverse masters unless I need to and when I do I don't abuse the privilege. Don't blame the messenger when it becomes clear that really bailing a leaky boat with a sieve doesn't work. Go for logging, such as cams (a single good lock would pay for it) if it matters or deal with that locks aren't that good.