Is it possible to change the "Install" button display text based on some condition i.e if fresh install then "Install" if upgrade then "Update"?
Basically, on major upgrade i am skipping some of my custom dialogs (which are shown on fresh installation) so on InstallReady dialog i want to show "Update" instead of "Install" as the button text.
Yes, you can do that but you'll need a custom "VerifyReadyDlg" dialog. You can just show the "Upgrade" or "Install" button depending on the upgrade/install like it is done for other options which present there (Remove/Change/etc).
Just check out the VerifyReadyDlg (the built-in wix dialog).