I believe you would want to send TTM_DELTOOL message to your existing tooltip control to delete the current text, followed by a TTM_ADDTOOL message with the new text. Alternatively, I think you can use TTM_SETTOOLINFO to modify the text of an existing tooltip - that may be simpler. The following snippet shows the syntax for TTM_ADDTOOL and may point you in the right direction, although the code is WTL instead of MFC:
TOOLINFO info;
ZeroMemory(&info, sizeof(TOOLINFO));
info.cbSize = sizeof(TOOLINFO);
info.hinst = \_Module.get\_m\_hInst();
info.uFlags = ( TTF\_CENTERTIP | TTF\_ABSOLUTE);
info.lpszText = \_T("Your new text here.");
m\_Toolbar.GetItemRect(m\_Toolbar.CommandToIndex(ID\_TOOLBAR\_BUTTON\_TO\_CHANGE ), &info.rect);
info.hwnd = m\_Toolbar.m\_hWnd;
info.uId = ID\_TOOLBAR\_BUTTON\_TO\_CHANGE;
::SendMessage(tt, TTM\_ADDTOOL, 0, (LPARAM)&info);// tt is the HWND of the tooltip control
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