So, your question is a little off, because propeller thrust changes dramatically with airspeed. Propeller aircraft are typically referred to as being "power producers", i.e. we talk about power when we analyze props, not about thrust, because the mechanics and mathematics behind propeller thrust are annoying and complex.

But to get to the heart of your question: You want to reduce the prop diameter (ground clearance perhaps? The engineer in me asks why you don't just make the landing gear taller) and not change the engine. You don't actually have to add blades (maybe). You could also just make the existing blades fatter, or change the airfoil, or increase the propeller RPM. Lots of ways to attack that.

But, playing along that adding blades is the only way:

1) Take your existing high-wing plane and calculate power required for all phases of flight: Take-off, climb, cruise, etc.

2) Take your new low-wing plane with its smaller prop diameter and work backwards to ensure you can actually meet the power requirements to stay aloft through your envelope. Adding blades reduces efficiency because the blades' wakes interfere with each other so you'll have to dig into some experimental data based on the prop of your choice. Much depends on blade pitch and washout.

Very likely you will need to increase the RPM (if your engine can deliver enough power) or change engines to a more powerful model because your props are now less efficient.

Such is the nature of aircraft design - almost nothing can be changed without affecting something else.