what do you mean?
A threaded screw on freewheel or a HG type cassette that slides on to a splined freehub fixed to the hub?
I once bought a cheap threaded freewheel expecting to remove the cogs and swap for the shimano cogs of a shimano threaded freewheel that had better ratios, I found they used a different spline. I don't know about the ETC brand specifically.
However any HG cassette will be interchangeable. (bear in mind 7spd are narrower than 8+ spd).