The Tokyo High Courts ruled the House of Representatives elections in 2012 as unconstitutional, though the results as non-unconstitutional.
…meaning that the HoR needs to fix the malapportionment, but the MPs elected in 2012 can maintain seats.
If the group appeals to the Supreme Court and the SC would rule both the elections and the results unconstitutional, then I don’t know what will happen, but here are some options:
1. The HoR will be dissolved. It could be argued the entire chamber is unconstitutional.
2. The HoR will have by-elections but only in under- and over-represented districts. Or probably only in SMDs. At least MPs elected on PR should be constitutionally OK.
3. The HoR will fix the rule but won’t have the by-elections, because it could be argued overturning voters’ decisions, however malapportioned they were, should be undemocratic and unconstitutional.
The link to the story Kuni is referring to (in Japanese): Asahi.com.