I am renovating our kitchen and knocked a wall out. Around 6 electrical wires in the wall had to be lengthened for re-routing. I joined these in the attc in junction boxes using 14/3 Romex.
Now I have come to learn that the circuits are 20amp. The original wire was 12/2.
Should I change this out? A friend seems to think it will be fine. If it's a problem I won't hesitate in changing it out if it's dangerous.
Thank you for any help.

Yes, change it. 20amp requires #12 wire.

Yes, change it. 20amp requires #12 wire.

Can this be dangerous?

Yes, it's a fire hazard. #14 wire is only rated to handle 15 amps, 20 amps can cause the wire and insulation to overheat and subsequently melt.