The King's grandchildren who now have lost their HRH-status don't have surnames. They are e.g. Prince Alexander, Duke of Södermanland.
Glenn, you seem to have stumbled onto the answer. Presumably the King's non-HRH grandchildren will be addressed as Prince or Princess X if that form is already in use for HM's non-HRH sisters.
Johan, you raise an interesting question.
It would be so much easier if the Court actually thought these decisions through in advance - and communicated that: and they will now be adressed as...
As the Court has not communicated anything - then we can only guess - for now.
The only thing we can rule out is: Mr./Ms./Mrs. as Mr. Prince sounds very strange.
In Sweden you only have the following prefixes to choose from (male): H.M., H.R.H. (H.H. if you mention foreign persons with this prefix), H.E. or Mr. and none of these seems right to use for the King's grandchildren who lost their H.R.H.
I think the Court will look to the King's 3 sisters who also lost their HRHs - as examples - and here one just avoids 'the issue' by adressing them as e.g. Princess Christina... and the polite form of 'you' when speaking to them (some would pershaps even use Princess Christine when speaking to her, but personally I would never do so).
When adressing nobles in Sweden - you still only have the prefixes I just listed to choose from - and if the person you are adressing does not fit into one of them, you simply does not use any prefix.
So when writing to a noble (you have both nobles with or without titles) you just write: To Count XXX - and when speaking to them - again the polite form of you… or some might say: would the Countess like Another glass of... but again not many would choose to do so.
RW1