No. You need to make sure all wiring connected anywhere in the circuit is also 12 gauge, and all devices, switches and receptacles should be rated 20 amps as that is the current available to them from a 20 amp breaker.
I just had a new electric power panel added to my electric service.The electricians never turned the power off, but they are professionals.You will have to know what outlets the new 20 amp services, because they are also rated for either 15 or 20 amps.They should be upgraded to 20 amps. Also you do not say why you want to change to 20 amps. 15 should be enough. (01/18/2007)
Please, get an electrician to do this for you. The fact that you say it is an oven tells me it is 220V. This should be a double breaker. Most ovens run about 60 amps which would be a double 30 amp breaker. You cannot change breakers without first knowing the amperage capacity of the feed wire. This is really not a DIY project if unsure about all the facts. (09/24/2007)