Being a Rails update expert, I would like to suggest updating the ROR version. It will cost ZERO.
Did you hear correct! YES.
It will not cost you if done on time with a good resource with proper plan. Otherwise, It cost your customer trust which is invaluable.
Sometimes if the project running well on the live server with a handful of customers without any issue, rails update is taken less seriously.
also, Rails update is not considered as important task. But it may cost a lot if website may go down due to unsupported version of any third party dependencies mismatch.