first of all, find the agent that u want the new maid to be, the agent will match the time with you and prepare all the documentations.
i just did that to my bro, my bro's maid:AA has been under the agency xx, then we changed AA TO my agency zz, 1st ask the maid to get back her passport fr her original agency((it's illegal to keep other ppls ppt including the agency)) , in this case the new maid's of her current employer will get noticed by the agent that mean she will get fire at any time . once she got fired she can stay in hk for 14 days to look for a new employer(u can let her stay at yr house ) ,after 2 weeks she need to go either macau or china for the SHORT STAY( waiting for the new visa).
it's not difficulty at all, but bear in mind that u need to match the time with yr agent(i m sure they will teach u what to do) , at this stage what u need to do:
everytime the maid changed emplyer, they still need to pay around $3000 to the agent for a handling charge AND one of the reason why they a keeping it.