keep "customization notes" for each client, the same way we keep changelogs. it makes it easier to find what was changed and when. then review the notes when upgrading or troubleshooting for anything that your changes may break.
a good plan is to also submit these customization notes with your invoice to the client.
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Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.