I live in Louisiana and I have a Lennox heat pump unit at my house, the unit was there when I purchase the house in 1995. My question is will I have to replace the inside unit if I get a bigger unit outside? Any idea what something like this will costed, one service guy told me it will be around $5,000 to $6,000, to replace both.

Thanks in advance.

Evaporator/condensor coil located inside yes. The air handler, no.

Coils and outside units are matched.

is according to how much bigger,,if you might have to replace the indoor blower motor,it needs to come close to matching the outdoor unit,,,what I`m meaning you won`t get all it can give you if you have a 3 ton condenser and only a 2 ton blower

In that case it might make sense to evaluate the duct design with a Manual D calculation. Manual J does sizing.