Series 3 Tax exempt do's and don'ts

Morning crew.
I work up in a cold sweat pondering what have I done.

Will putting on a like for like new chassis mean I loose tax exempt status?
Then I thought, what about the other things I wish to do?
Power steering?
Led lights?
Disc brakes?
Will this affect the tax exempt status?

The engine, suspension and gearbox are staying original.

I read only substantial changes affect the status, but what constitutes substantial?

Yours, a now worried Shirl.
Btw, it on a 1975 lwb s3 station wagon.

No for the chassis as long as its "as it was" its fine. Get the exemption now. Once you have the exemption It will be an easier.

This might help
Vehicles of historical interest (VHI): substantial change guidance (publishing.service.gov.uk)
You are fine with chassis swap plus other proposed changes are arguably "safety related". Power steering might be a moot point but realistically, is anyone actually ging to check??
I have a couple of other classics that are exempt, both have brake and suspension upgrades but wasn't a problem to register them as historic. My S3 has replacement chassis but will certainly be applying next year when its eligible for exemption.
Cheers..Ian