Can you use the new refrigerant with the existing inside unit?
A process called flushing is done to achieve this - the inside coils (Evaporator) AND the lines that feed the Evaporator are cleaned out with a solution - it is also done with nitrogen. The flushing not only removes old refrigerant, but also the old oil which can be detrimental to the new compressor.
There are some caveats to this as pressures of the new refrigerant will be higher and therefore you will face the possibility of a leak and then need to replace the unit.
My outside unit is a 3.5 ton unit but I am going to need a 4 ton due
to an expansion of an added room to the home.
[Do you really need to upsize your unit Sizing Calculator at this Link uses the J-Model Sizing Calculations]
Now here is the issue you have with upsizing your unit.
The inside unit is most likely a 3.0 / 3.5 ton evaporator and normally an orifice is what changes the performance capacity for the evaporator.
You will not be able to install a 4 ton unit and expect to have 4 ton cooling capacity EXCEPT you install an Evaporator that is capable of 4 Ton Capacity.
Also upsizing you have another issue depending on your unit and the piping originally done - you might not have the correct pipe diameter from outside to inside unit (called a line set) - in which case the line set will need to be replaced as well. (if they don't do this - you will have issues (a higher electric bill as well as wasted money on a 4 ton unit operating at 3 ton levels).
Here is my advice:
Replace both the inside and outside units with a new unit - and also the line set going from inside to outside unit - if you spend a little more money and get a 16 seer unit - there might be tax credits available, not much but perhaps enough to Just offset the cost difference. You can also look into the utility company and see if they offer credits for more efficient units. Some units can run on Gas lines - so the possibility is the gas company might give you a break (or the furnace for that matter gas/ electric they might give you a break for using a gas furnace - perhaps even pipe it up for you (they expect a customer buying gas instead of electricity.. crafty way to get new business. sometimes the state will give you a credit for an energy efficient unit.
Depending; you could get a little or a lot, now if you only get a
little how does it sound worth it if you only get tax credits that
basically is a wash out?
Your electric bill will be lower (how much depends on how cold you keep the house and how hot it is outside) but expect at least $15/month ($150/yr) aka known as Grocery money.
A Typical 4 Ton unit from a reputable AC place will run you about $5000 to $10,000 .. I have just replaced a complete unit / except the line set [3.5 ton] for a property in another state the total was $5400. If I had replaced the unit here in the home I live in the price tag would have been $3500. There can be big differences so shop around.
Here is what these AC companies like to do - they like to schedule to come out (they do have good reasons for this - but more on why you should not do that up front).
That is the sales pitch and the pressure to sign on the dotted line as well as how many people you would be willing to have come out and give you an estimate of cost?
I would venture most people would have 1 or 2 come out and rarely a 3rd..
DON'T let them come out - just tell them you want to know the rough cost over the phone for replacing the entire unit Air Handler/ Evaporator , Condensing Unit and Line Set with a 4 Ton Unit. Get the rough prices and the brand and types of units - try to keep the same type of unit and SEER rating across the board with all of them.
After you get that rough phone quote (some might not be willing to rough quote - just go ok and move on) - select 3 of them that gave you a rough quote and have them come out and give you the complete written estimate from a home visit.
I called around many people would not quote anything over the phone and insisted to come out (I am sure I would have had the $8,500 to $10,000 quotes at that point) along with the sales pitch and the yes ok I will work with you and drop to $7500.. after getting my [3.5 ton] unit installed / furnace ($5400) an ac person told me I got a REALLY good deal and for what I asked for I should have been hearing $7000 and up and he was right that was what a lot of them told me.