Transplant question

Hello growers! I transplanted clones from solo cup to a 5 gallon. I am beginner grower. Plants are 4-5 feet and 3 weeks Into flower. The bottom leaves keep going yellow. People have mentioned that I should transplant to 10-15 gallon. My question is do I stick it out with the 5 gallon this season and next year do it right? Or should I transplant 3 weeks into flower. When I look at the bottom of my planter I see roots. Thank you.
 
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