At the time, ...Forever was viewed as the third Batman film, the sequel to Batman and Batman Returns. But I think that had it been made today, then it would have been described as a reboot - different actor, different director, different tone, different look for Gotham, different soundtrack, different Batmobile, different costume, etc.
Except it carried forward the same Alfred and Commissioner Gordon. As far as I know, the only series reboot that's brought back an actor from the former incarnation in the same role is the James Bond reboot (with Dame Judi Dench). Unless you believe the security guard Lou Ferrigno played in The Incredible Hulk
is the same one he played in Hulk