Yeah, I don't see a problem either - unless there's a mad end to the season, you'll have Weymouth & Dover heading down into NLS and King's Lynn heading down into NLN - and as NLS played with one fewer team this season, that's all hunky dory.
And then they're promoting eight teams from Tier 7 this year, and I believe they're doing two each from the Northern Premier League, Isthmian League, Southern League Central and Southern League South.
But given that all the teams in promotion contention in the NPL and SLC are further north than Oxford.....I don't really see why City should have to move leagues?
It seems to have been discussed as likely in non-league circles (generally decent websites/Non-League Paper etc). It will as you say though depend on those finishing in promotion/relegation spots and that won't be known until later in the season.