It is often possible to use the "present perfect" (I have changed) or the "past simple" (I changed). Just like your examples.
But do not use the present perfect when you talk about a finnished time( for example, yesterday / ten minutes ago / in 1985). use the simple past:
1.I 'lost' my key yesterday. ( not I've lost)
2.It 'was' very hot last summer. (not has been)
3." 'Did' you see the news on TV last night?" "No, I 'went' to bed early."
Do not use the present perfect if there is no connection with the present. :
1.The Chinese 'invented' printing. (not have invented)
2.Beethoven 'was' a great composer. (not has been)
Compare present perfect and simple past:
We use the present perfect for a period of time that continues from the past until now.
past [----------- l ------------] now
We use the simple past for a finished time in the past.
I hope it's been useful. ;)