I have an exsisting 400 AMP service with 2-200 AMP sub-services, can I replace one of the sub-service and replace it with a new 400 Amp sub- service without replacing the incoming feeder cable from the outdoor transformer?

Are you asking if you can replace a 200 amp service wit a 400 amp and a 200 amp being fed by a 400 amp supply?

Will both of the new services be at full load?

Are you asking if you can replace a 200 amp service wit a 400 amp and a 200 amp being fed by a 400 amp supply?

Will both of the new services be at full load?

No, they will not

I'm not sure what you are asking. If you are asking if you can replace a 200A panel with a 400A panel the answer is no.

What do you mean by sub-service?
A 400A residential service with two 200A panels is pretty common these days. The 200A panels are not sub-services. They are both service panels.

I'm not sure what you are asking. If you are asking if you can replace a 200A panel with a 400A panel the answer is no.

What do you mean by sub-service?
A 400A residential service with two 200A panels is pretty common these days. The 200A panels are not sub-services. They are both service panels.

This is 2 industrial unit, with 2-200 amp sub-service supplying power to the units, both 200amp is being fed from the exisisting 400 amp main service, can I replace one of the 200amp with a 400 amp sub-service.

No, you cannot.
Please call an electrician.