Thnks krishna for your help
i made a stored proceduren i checked the syntax, it was okay:

create procedure sp_priorites

as
execute immediate
alter table dbo.priorites
drop constraint pk_priorites
go

execute immediate
alter table dbo.priorites
drop column _codepriorite
go

execute immediate alter table dbo.priorites
add _codepriorite int identity
go

execute immediate
alter table dbo.priorites
add primary key (_codepriorite)
go

do you think it might work this way?