I get a kick out of some of you holdover "I bleed IBM-blue types" . It's 2021 and you'd be out on your hindquarters if IBM thought it in its best interest. More RAs are coming.. your number may well be up.
I've left several companies on a Friday to start a new job Monday. They would terminate an employee the same way. What's good for the goose is good for the gander. And I got hired back as a consultant for a few of them in the ensuing years.
Conversely, I was also a "good company boy" and gave 2 weeks notice to other companies. They still took my resignation as an affront and would never consider re-employing me - some dopey company "policy".
I've consulted for IBM over 1/2 dozen times in my career. A few times, they stupidly let myself and other contractors go en masse- only to recall us some months later. No problem here - but I did hike my rate. Stupidity is costly.
So - it' s all a lot of El Toro C-apo. Take care of yourself, your family and your career. That's the SMART way of staying in the black $$-wise. And try not to ever get too attached to a company or job. It's a quite unnecessary career detriment these days.
If I were you (and I am not) I'd actually start the new job on my IBM vacation time. If the job is looking like it will work out, then just call, email, or Slack your boss and give him the adios. If the new job stinks, well then, just go back to Big Blue from vacation and repeat the process. You are aware that it's still a (relatively) free country? Worried about a "bad reference". Don't worry. IBM will only confirm your employment dates -nothing else will be said or the smell of lawsuit enters the air. In light of the well known RAs going on at IBM, no employer is going to bother you about jumping ship from IBM.
So - get focused on priority ONE - YOU.