If there are inline strainers then generally there are isolation valves near to it, When the strainer is put back in the trick is to open one of the valves a little to flood the filter chamber area and push the air out before doing up the door to the filter strainer. Slight loss of pressure is then topped up with the filling loop.
Changing a circulation pump will invaribly introduce air into the pipework. In many cases it should be actively purged out. Whilst air will rise to the highest points of an installation where air vents should be installed, there are often places where air will be trapped and needs to be forced out by a big circulation pump.
One of these sort of things is a professional cart. One of the plumbers in our company makes his own cart. Take one small wheelie bin, one huge Clarke sump pump and a few hydraulic fittings and a flush cart is made. Connect it to the main flow & return points that are tee’d into the installation and it will be able to flush out debris and air.