Is a chimney liner needed when replaceing an existing gas boiler
With a new one?

Is a chimney liner needed when replaceing an existing gas boiler
with a new one?

Each different brand and model number of boiler might be different. I always suggest following the manufactures install instructions since those have to be followed in order to make the install safe. With that said here are some other situations for your boiler.
You did not give the brand or model number of the unit you are going to use.
You did not say if it was a inside or outside chimney.
You did not say weather it was condensing or non-condensing boiler.
Chimney height could come into play also.

You are missing a lot of info and without it no one can tell you for sure if you need a liner or not.

My rule has always been to follow the manufacturer instructions with no modifications unless you can get the boiler manufacturer to sign off on the modifications and that is just about impossible due to liability insurance.

Yeah, need the info. Mentioned by hvac1000 and also would like to know if the new boiler will still use this chimney and if so or not, do you have a water heater that will remain on this chimney? More info. OK?

Mark