The BL51 is a great table but so is your current MR611. I don't think you would notice an improvement worth the $.
If it were me I would look at a cartridge (or an arm) (or both) for your current table rather than possibly an arm/headshell/cartridge/armboard for the BL51.
MS armboards are scarce and cost a small fortune.
To some extent I do agree with this sentiment as we both know how the MR611 is a great TT. I suppose I have a bit of an itch to at least try and upgrade, the BL51 would hold it's value, it does have a nice arm and I think it would be easier for me to change the arm on a BL51 as I can remove and replace the arm mount. I don't have the guts to replace the arm on the MR611 like you did, not really a removable arm board like the BL51. Also the BL51 is almost 10 years newer. I hope I'm not just convincing myself here!
In any case the MR611 would go into my daughter's room so it would still be regularly used. Not like it's likely to break down in my life time.
I've had 20 very good years after buying the MR611 second hand, I can hardly complain. It would also mean I have 2 Micro Seikis in the house
My System: Micro Seiki BL51 Turntable, Stax UA7 Tonearm, RCM Sensor Prelude Phono Stage, Ortofon MC15 Super II Cartridge, Rotel RCD865BX Cd Player, Melody I34R (Astro Black 40) amp and Osborn Epitome Speakers.