tyre

This will probably sound totally dumb or silly but we all ask dumb questions when we're noobs so bear with me and hear me out. Coming to the sport late, but better late than never. I was way into cycling a few years ago and started riding my bike (a Centurion CitySpeed 8) on a regular basis...

I have a Firefox bike that came with 26" double wall rims and a 26X2.10 tyres. Obviously the 26X2.10 tyres are meant for mountain biking. Now, I'm planning to change my tyres to 26X1.5 and I wanted to know if it is possible at all. Do I have to change my rims to accommodate the slim tyres or so?

Tread in the middle is almost gone, but the tyre still rides fine. Total distance travelled on it is only 1300km, but the rubber seems to be a soft/grippier type and less hardy. Should I change it pre-emptively or keep going? How do I know when it's time to change?