Can I at the first time qualify the water system without following 21 CFR part 11 recommendation and do another requalification as soonas the delta V system is installed and validated? [/quote]
Can you? Yes. Would you be exposed to regulatory repercussions? Yes!
Instead of sweeping it under the rug, though, do a risk analysis (documented) on what not being validated for ER/ES might result in and (likely) take some precautions (e.g., print out paperwork temporarily).
Can I take as reference test done in the SAT during this month?[/quote]
Depends on several things. Did the SAT fully verify the system (demonstrated with objective evidence that all requirements were met)? Was the SAT done in a production environment? Do you have procedures (e.g., a Validation Master Plan or Validation SOP) that would disqualify SAT results from being used?