Hey is it O.K. to change a 100 amp main breaker on a 100 amp panel to 125 amp breaker?Iwas told as long as the panel is rated to 125 amp it would be all right. I don't feel comfortable doing this.will this be all right or is my gut right. This is a 1-phase panel were talking about, Just want to be safe and 100 % sure!

The wire also has to Handle the current.
Aluminum or Copper?
http://www.mikeholt.com/technical.php?id=nec/unformatted/poster&type=u&title=Commercial/Industrial%20Wiring%20and%20Raceway%20Chart

Hey is it O.K. to change a 100 amp main breaker on a 100 amp panel to 125 amp breaker?Iwas told as long as the panel is rated to 125 amp it would be alright. I don't feel comfortable doing this.will this be alright or is my gut right. this is a 1-phase panel were talking about, Just want to be safe and 100 % sure!!Told by whom??
What would you possibly need to do that for anyway? If you are that close with your demand or actual load then an upgrade to 200A would serve you MUCH Better.

As Strat said, the service or feeder conductors would need to be sized properly to even consider it. If it is a 100A now there is about a 99.9% chance the service is sized for 100A.