The plumbers I use charge $75 on the high end to swap out toilet parts or shower diverters. 

Are you swapping out the furniture?  

General cost for a Furnace and Condenser swap out?

What people say is "investment properties have higher rates," what they mean (without realizing it) is "investment property mortgages have way higher fees, which can be offset and swapped out for a higher rate instead if you so desire." 

Where I’m at they don’t even care if it’s knob and tube no one even requires you to swap it out .

I was thinking of swapping out my nicer stuff with Ikea stuff for them.

The GC on one of the projects we have in progress was way out of range on several trades and I told him that, so he price checked and ended up swapping out about half of his trades and we lowered the price by over $40k.

But if it is simply an inplace swap, my AC guy generally charges about $3K, but I do about 15+ swaps a year and tons of service calls. 

Is it worth swapping to one of these combination units or just fixing the gas boiler for now and replacing the water heater?

I use that an excuse to go inside every single property every three months and the tenant likes knowing that they get a fresh air filter.During those filter swaps you can find out if they are messy, cooking meth, growing pot, or damaging the property and get it taken care of before it turns into a multi thousand dollar repair.