If you have the time and energy, of course you train the new maid. As you said the standard will be higher so there is more room for lower performance later. What's the quality of a new maid who is trained by an old maid with low performance? You should know the answer, right?
Also, it also depends if you have confidence in the relation between you and your old maid (wether she is going to be on your side or the new maid's side). If the old maid is loyal to you (since she have worked for you for 7 years), she may put your interest in the first priorities. Otherwise, she may teach the new maids tricks. Afterall, maids bond very well because they think they should help out each other in a foriegn city.
If taking leave is not very possible for you, the only choice is to rely on your existing helper to train the new helper, but of course you have to bear the risk!
I just went thru such transition before CNY. If I could have choice, I'd rather fight for taking a longer leave to train up the new helper on my own. Because 舊工人 must take the chance to teach 新工人 how to take the best advantage from employer!! My instruction for 舊工人(bun, >10 yrs experience) was to train 新工人 (yan, brand new to HK) all the rountine househouse chores and to take the best care of my kid. At the end, I found out 舊工人 already put a "curse" on 新工人 to regard her as a reliable big sister. 舊工人 even asked 新工人 to bring back the story books returned from other helpers in the neighbourhood. I was very mad when someone passed the story books to 新工人 in front of me. I grabbed the story books and told 新工人 to only perform my scheduled duties during working hours and it's absolutely unacceptable to do any other personal things which is not related to my household. To correct my 新工人 and for her own good, I have to talk/teach/warn her not to contact 舊工人 again.
Fion_Chan, my advice is not to have such transition. It might turn into a nightmare!
I have this experience, I would suggest no hand over training.
I would agree with other two mums comments. The most important thing I found is.
When hand over you have two maids in the house who are sharing the work, but when the old mail leaves, the same amount of work will need to be done by the new maid who may already get used to sharing only half of the work, the new maid will find double workload after the old maid left and her standard will even be lower than first trained.