Look at the worldwide delivery dates of the 12" MacBook on apple.com (and apple.com/uk apple.com/de etc) and you will notice that the 'old' 12" MacBook is only 'available' for delivery on Wednesday while other Macs are available for Tuesday delivery 😉 That means that there will be a refresh of the 12" MacBook series on Tuesday I guess.
This is the most likely outcome, except I think if it happens, it will be on Thursday or Friday, not Tuesday.
Where I am, you can get almost every type of Mac, both standard and customized options, delivered by Wednesday using the "fastest" shipping option.
However, the only option for the rMB is standard shipping, delivering on Friday. Absolutely no option for shipping besides "Standard".
I suspect refreshed models are probably on their way to Apple stores already, and it will be a silent update. That means most likely just a refresh of the CPU. Anybody who orders one today, will likely get a refreshed one on Friday in the mail. The newer CPU will mostly be noticed with an increase in battery life. Some may like the marginal bump in CPU and GPU, but most probably won't care (they are more likely to care about their battery lasting longer though.)
That other metal injected hinge business being spoken about elsewhere ... I doubt it's for a redesigned rMB. I suspect that if true, it's more likely to involve either a redesigned Air or Pro that will probably be announced this summer.