I heard from the Symbioz forums a few days ago that the STIF is proposing to re-deploy twenty trainsets of the MI 84, the commuter rail rolling stock that is gradually being phased out from the RER Line A. This latest development appeared in a media release from the STIF dated February 13, 2013. The release is in French, so you will need a translating service to translate the document into your native language.
Until new rolling stock is ordered to replenish the aging fleet of trains along the RER Line D and Suburban Rail Line R, the STIF is proposing the use of outgoing MI 81 rolling stock to reinforce a portion of both lines for the meantime. The sections that would utilize the MI 81 include the Melun/Juvisy segment of the RER Line D and the Melun/Montereau branch of Suburban Rail Line R. The MI 81 stock along the RER Line A is currently being replaced with the new MI 09 stock, which was introduced in 2011.
Some in Paris may wonder why the MI 81 is being replaced before its predecessor, the MS 61. My understanding is that the MI 81, though similar to its counterpart on the RER Line B, the MI 79, the MI 81 was constructed in a cheaper manner than the MI 79, resulting in greater wear and tear. As a result of this, none of the MI 81 stock have been heavily refurbished, unlike the MS 61. However, it seems that the STIF can put some of the MI 81 trains to good use for a few more years because they will not need extensive refurbishment to run on the other lines, probably just some minor aesthetic and mechanical improvements.
In addition to the possible movement of the MI 81 stock to the RER Line D and Suburban Rail Line R, the STIF is also proposing to bring forth 12 Z2N commuter rail trains to the northern portion of the RER Line D to strengthen service. These trains would be transferred from Suburban Rail Line P, which is currently getting the brand new Z 50000 series rolling stock (known also as the Francilien).
HARTride 2012
The Global Transit Guidebook 