You can retain your ruby status - for what it's worth when you aren't flying MH, since it doesn't give you many benefits on other oneworld airlines* - until the end of the membership period, and accrue miles on AA during that period in the hope that you get status with them before your existing membership expires. Bear in mind that whatever level you reach with AA, it won't get you lounge access on US domestic flights because these are carved out under AA and oneworld rules

* Although you can still "mix-and-match", in that you could use business class check-in using your existing Enrich (ruby level) card even though you credit the flight to AA's programme. It is more difficult to get access to the other two oneworld ruby benefits - preferred seating and priority waitlisting - since you would have to have your MH number on the booking for this to work. However, you would then have to switch the FF number to AA shortly before the flight (and you should be able to do it at check-in), although at that point I suspect you will lose the priority for remaining flights in the same booking