No, generally you can't just substitute a bunch of pex tubing for your existing radiators or baseboard heat.
First, you need to know how much heat you need to supply, then that would help calculate how close together the loops need to be, how many loops you need, and the diameter of the tubing, then you need something to temper the water temperature, since baseboard or radiators typically use 180-degree water, which you can't put through your floor - talk about hot feet!
You need the supply temp much lower, and deal with the max floor temp based on what covering is on the floor, the supply temp, and the flow rate. You want each loop to be approximately the same length, or you will need to have valves to balance them, or some will get more flow than others. It's not a bad idea to run the loop around the outside walls first, since that is typically the coolest, then come into the main part.
There's a lot to designing a well-performing system.