I don't think they can do what is outlined in Trump's plan. All major changes to government employee compensation has applied to new employees, FERS, FERS-RAE & FERS-FRAE. CSRS Offset employees had special rules for those that had a certain number of years under CSRS.
Interesting article from earlier this year https://chiefhro.com/2017/01/10/6-civil-service-changes-we-can-expect-from-the-115th-congress/
I happen to have the CSRS Offset plan as I left and returned to Civil Service. Whoo that is a wild ride! When I started the Offset plan in 1987 there was only one lady in DC who could answer questions about the plan. All I heard was trust us it'll be fine! It actually turned out to be like FERS without the matching and the Supplement. You pay SS and Medicare on your salary and the net difference between SS and CSRS ( .8%) if I remember correctly. They then pay your regular CSRS annuity less the SS pension you draw based on your Offset years. Then you receive a Soc. Sec. Pension as adjusted if you have less than 30 years of SS earnings including your Offset years. Exciting enough for you yet? Also, you have to draw your Soc. Sec. at 62 whether you want to or not. They still reduce your CSRS annuity by your Soc. Sec. receivable at 62 whether or not you actually received it yet.
Watch the changes but don't freak out. It may change a thousand time or not before the budget is finalized. They have been trying to cut our pay, benefits, and retirement for the last 40 years through Democrat and Republican administrations alike. My WAG is that major changes will impact new hires only.