In my experience, the Renaissance never upgrades anyone, not even this longstanding PP, from Barlow to Chambers rooms without an extra charge, usually in the neighborhood of £100 per night, though the offer has been known to vary. I do not know if SNAs will change this longstanding policy, but given the tenacity with which the Renaissance has clung to it, I would not expect an upgrade from Barlow to Chambers. I have received upgrades WITHIN the Barlow wing, but the difference between the various Barlow rooms is negligible. If, however, you book a Chambers room, there appears to be a decent chance of an upgrade to a better Chambers room.

As to rollaways, none of the Barlow rooms allow a rollaway bed. Chambers rooms do. As an FYI, this information is available on the Marriott website. When booking you simply click the "room details" link to the upper right of the room type you are viewing. Availability of rollaways is listed under "Beds and Bedding."

As far as lounge access is concerned, you currently have access to the Chambers Club due to your Platinum status (by linking your accounts), but it is unclear if this will continue in the new program. You may want to call or write the hotel and ask. This hotel does NOT consider the Chambers Club to be a concierge lounge. They call it a private club. Currently MR Golds are treated as if there is no lounge and are given breakfast in the restaurant. It is entirely possible that this becomes the policy for Platinums going forward, and possibly for PPs. We simply do not know.

Finally, regarding under-12s, please note that the Chambers Club does not allow access to guests under 12 years of age. I am not certain how strictly this is enforced.