I got a new Platinum Travel Card today. I actually applied for upgrade to my MRCC, then got a call from Amex employee, who suggested if I were interested in keeping MRCC, then I should go for a new card instead. Since I was curious, I asked if I can hold MRCC as a companion card, to which, the Amex employee assured me that my existing MRCC will be internally converted to Companion card and it will stay that way as long as my Platinum travel card is active.
Cannot confirm just yet, as my upgrade request also got fulfilled and as of today, I have 2 Platinum travel cards, requested them to revert the upgraded card back to MRCC, post which the companion card process will happen. Will update If it actually happens.