Questions about current version

Hi, I wrote so many lyrics about 1.5 years ago and made them into songs using Suno v3 or v4. I even made lyric videos and uploaded them in YT. However, I have stopped my subscription since then and I really want to improve the quality of these tracks. My questions are:

1. Can I improve the quality of the songs but keeping the melody intact using the latest version?

2. Can I just change the voice but keeping everything else the same? I want some songs to have the same ‘singer’.

3. Does the singing voices still sound robotic or are they more natural now? Should I just wait another year maybe for a better version?