It's complicated... first of all download the T&Cs below...

https://www.accorplus.com/~/media/fi...s_2017_gb.ashx

... then go straight to the last page

APPENDIX 1 – ACCOR PLUS MEMBERS
If you are an Accor Plus member, the Status assessment will be different and based on the following rules, commencing 1 January 2017:
At the time of your Accor Plus membership renewal, the Status will be assessed based on the Eligible nights or Status points earned during one of the two periods:
1. The current calendar year
2. The previous calendar year
The highest number of Eligible Nights spent or Status Points earned during one of these two periods, will determine your new Status.
If, during the previous calendar year or the current calendar year, you did not reach a threshold allowing maintenance of your current Status, you will have a Status corresponding to your activity during these two periods, as indicated on the Le Club AccorHotels scale (refer to table in clause 8).
Accor Plus Members automatically qualify for Silver Status and are never downgraded to Classic Status, provided the member renews their Accor Plus membership.

So - say you are Accor Plus Platinum until the end of November 2017 (end of your Accor Plus membership) - comes 30/11/2017 they will look at Status Points & Nights for 1. Calendar year 2016 and 2. The period from 1/1/17 to 30/11/17 and use the highest number to determine your status for the following year (from 1/12/17 to 30/11/18). This is ok if your Accor Plus membership expires late in the year (e.g. expiring 31/12 you basically have 2 full calendar years to play with, expiring 30/11 you have a full calendar year and an 11 months period) but obviously not so good if your Accor Plus membership expires early in the calendar year.

What to do? Check now your qualifying Status Points & Nights achieved during CY2016 (you'll find this info under My Loyalty Card > My Points History) - if you have enough Status Points or Nights to maintain Platinum (or whatever status you are trying to maintain) then there's no need to stress, you will maintain your status at renewal time... if not, then you better start booking away if you want to maintain your Accor status!

To make things even more complicated - it looks like the above only applies if you decide to renew your Accor Plus membership... if you don't, who knows what will happen as you'll be left in limbo between the Accor Plus and the LeClub "systems"