For example, if I have 200k in my RA at 55. This will grow to 230k when I’m 65.
At 4% interest compounded annually $200,000 at 55 turns into about $296,049 at 65. There's another $900 per year of bonus interest (which is also compounded annually), so you end up with over $300K at 65.
If by the time BRS is 100K. Then after I transferred 100k to purchase CPF LIfe Plan, I still have 130k in my RA.
No, for lump sum withdrawal purposes the BRS is fixed at 55. So if you can withdraw up to $93K (the 2021 BRS) at age 55 then that withdrawal option persists at that level until just before your payout start age. (Plus up to $5,000 at age 55 and up to 20% from age 65, inclusive of the up to $5,000.)
Basically just assume that interest must stay in RA, and you've got the basic idea.
I’m assume I no longer eligible to deposit cash into RA anymore.
No, you can shove cash into your RA as much as you want and for the rest of your life, subject to an overall limit of the Enhanced Retirement Sum that's based on principal (not interest). If you have reached the ERS limit with your top ups then every time the ERS is raised you get more room to add more funds. If you live to 110 you can do that when you're 110 if you want.
So my RA will become a 130k saving account with 4% interest, free to withdraw anytime but not allow to deposit. Correct me if I’m wrong.
You're not quite right. You can remove a portion of principal from your RA prior to payout start and with a sufficient property pledge or charge. Interest stays in RA. You can add funds any time you wish, up to the ERS (a limit based on principal).
Can I choose to transfer 279k to my RA which is ERS at 55. Then at 70, I decide to opt for BRS?
You can certainly choose to fund your RA to the ERS, and to keep funding it to the new ERS every time the ERS is raised. Cash is usually best, and next best is if a spouse or other eligible family member transfers OA dollars to your RA (or if your SA is depleted but you have OA dollars). That's because a transfer from your own accounts draws from your SA first, and your SA is already earning 4% interest.
Your BRS lump sum withdrawal amount doesn't change. If it's $93K (total) at age 55, it's still $93K before payout start. Top ups and transfers don't increase your optional withdrawal amount. Of course top ups and transfers boost your monthly payout amounts after you start payouts.