i wasn't sure about changing them but after doing the job i don't think i've done any harm by doing so
No that was a good move considering how corroded the threads look. It it sits in the head another 2-3 more years could be tough to get out. This is exactly why I am going to pull mine soon at 72K.
They are not corroded. It's normal to have plug threads brown due to coked oil/grease. But waiting too long can clearly cause some problems.