Provided you only bung the feed pipe from the F/E tank. If you bung the vent pipe as well, air will not be able to enter the system to replace the water as it leaves. Make sure any motorized valves are set to MAN until after you refill the system.
Do it properly, turn off feed to expansion tank, open drain cock, remove/replace/refit new pipe work and refit rads. Close drain cock, turn on feed to expansion tank, fill system and remove air. Check for leaks. Turn on heating and test.
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