Learn English – say two-series to describe a series of two events

Since presumably (1) you're checking in something that you did, and (2) the change has already happened (otherwise you'd have nothing to check in), I'd go with the simple past:

Changed function argument type to int.

If you're checking in a change on behalf of a team, I could also see using:

Function argument type changed to int.

The simple past is the least wordy tense — no helping verbs, no "now"— and is generally the easiest to understand.

I wouldn't suggest using a continuous present or future tense such as "changing", because then the log becomes unclear about what was changed when. _{^{(Akin to the problem of distances in driving directions: when it says "left turn — 5 miles", does that mean the left turn is 5 miles ahead, or do I make a left turn now and then go 5 miles until the next step?)}}

You haven't provided enough context to say definitively, but if the phrase is a heading or title, no article is needed. Article “a” would be incorrect (although one might say “A maximum of variability and openness”). Article “the” would be clumsy. Note, if you are describing a software product, variability is not a positive feature. Also, some may prefer maximal to maximum.