There will be no implication for the existing VPN connections.
To answer your questions:
1) It will not impact your existing L2L tunnel
2) It will also not impact new L2L tunnel
3) Same, it will also not impact your remote users.
NAT-T is negotiated during the phase 1 negotiation. What will happen is if it detected that the remote user/L2L VPN is behind a NAT device, then it will negotiate the tunnel to use NAT-T (UDP encapsulated ESP packet - normally by default it's UDP/4500). If during the negotiation, it does not detect that the device is behind a NAT device, then it will continue to just use ESP.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...