i face the same problem as you before..
my ex-maid (finish 2 yr) and worked for a new employer, but they treat her badly, so whe want to come back and work for me again (i didnt employ maid that time).
But at the end, my ex-maid decided to work at current employer's place.
BUT I asked the agant about fee and procedures :
1. I (employer) need to pay the agency for 本地工人 (around $2xxx)
2. The maid need to terminate the contract with her current employer (in such condition, it is the same as employer fire her), so the maid need to go back to her own country and apply everything all over again to come to HK .
It takes the same time as employing a oversea maid (around 2-3 months), so in this period, the maid will be $0 income and eat herself
3. Also , because she cannot finish the existing contract, she need to pay 7 months salary to the school/agency to come to hk again. (like those maid first time come)
is it worth to do so? really need to ask you maid.