Suggestions for lightweight free software change control?

I'm looking for our first change control system for 5 developers working on software that is entirely written by ourselves. It's deployed to many customers and been in development for 17 years but is still being actively developed and rewritten.

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I'm the dev manager, but know little about change control systems. Historically there's been very little change control and we often have situations where it's not clear why something was changed in the past or what the limitations of new features are, so I am mostly looking for change logging rather than control.

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I've started by using a change control spreadsheet only for changes to the software that go ahead, just to test the waters, and I think this is enough (like i say, i want something very lightweight) but obviously I'd like it able to be edited by everyone at the same time. Possibly I'd also like to log change requests here as well.

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We do have Office 365 in case there is something offered by that which is suitable.