IANAL, just a fellow citizen trying to abide all laws and regulations, posting on the internet.
BLUF: It's not really clear yet. But you should probably take the brace off and dispose of it. Definitely by the compliance deadline, but doing it before the register publication may be smart too. Also, you probably should not file a Form 1 for that gun.
There are different interpretations of the statement from ATF about it. In a section responding to comment, they discuss financial impact of the ruling due to 922(r) issues, and state that individuals "may" destroy or turn in the gun in the case described. They also talk about replacing pieces not curing a 922(r) violation, as assembly is the crime. So there is an interpretation that they are saying any pistol imported with over 10 foreign parts that had a brace was an SBR at the time, and 922(r) applies to rifles (not pistols), and therefore illegally imported. Therefore it can never be Form 1'd, and isn't even a legal Title 1 firearm, and must be destroyed. This was the interpretation of one group of firearms law advocates, and has made the rounds.
IANAL, but it's a tiny section in about 270 some pages, and has none of the clarity (or at least verbosity) of the rest. There is some lack of clarity about who is actually assembling, for example. It is likely to be challenged soon, as well -- seems like "ex post facto," even if it's not a law.
My opinion is that ATF promulgates rules for a wide audience, and this comment is likely directed at importers and dealers who have significant financial impact from an inventory of now non-compliant guns. They probably didn't think about it beyond that group. But it may inadvertently ensnare the average Joe plinker, who doesn't know what this law is, doesn't import guns, and never discussed any of it with his FFL, from whom he very explicitly purchased a pistol (922(r) does not apply to pistols). By changing the definition of your pistol to a rifle in this rule, the ATF has brought the entire chain of import and sale of your gun into question. So at least if you remove that brace now, before the rule change hits the register, it wasn't in that configuration when the new rules took effect. That may not get you off the hook, as the definition may be changed retroactively, or some other reason; I am still not a lawyer.