Bad news first: Those of us who bought the old expansions got the disappointing news (years ago) that this purchase was not transferable to Shuffle. Period. Full stop. So, it's a matter of paying over and over again, for each month that you want to play. Yes, we know; none of us was happy about that decision. But that ship has sailed.
Possible good news: We who had kept on playing got a free year here. (Well, not really free, since we'd already paid for a "lifetime" of Dominion [we thought] . . . but we did get a year with no monthly/yearly cost.) This has long-since expired, but if you contact Stef (the head honcho) and explain, you might get that free year. Can't hurt to ask, certainly.
Sort of good news (certainly not bad news): You can play for free here, but usually only with the Base Set, which of course gets dull right away. But I believe that if you are a free player, you can still match up with those of us who are paying for more/all expansions, and can therefore play games with more than Base. (I've bought the full set, so I'm not 100% sure about this...I've obviously never had to try and do this type of matching.)
Silver Lining: Since you will have to pay going forward, it matter less if your old account is, indeed, lost. :-)