Not necessarily ^^
I have one strain that always needs to be repotted in flower...DNA genetics Rocklock doesn't slow it's roots at all...lol...But then again it is by far the most compact plant I have grown...
And there is nothing wrong with transplanting in flower...it is not recommended because this is not the point where you want to mess up...but for real no worries...just cot the bag and plop it in the new pot...
fill the 5 gallon bucket with soil to desired level.
Set the 3 gallon bag inside the 5 gallon pot.
Cut the bag off of the root ball, leave the bottom there and remove the rest.
fill the rest of the 5 gallon pot with soil and water