Trying to figure out exactly what it is you're doing here ... are you using these sequences as nests (dragging into timeline window and dropping on track) and wanting to replace a nest already in the active sequence, or are you just opening an existing sequence for editing (and wanting this action to close the sequence that's already open in the timeline window automatically when this happens)?
If its the former then you can drag the clip or sequence onto the Canvas window's "Replace" overlay or button (or you can drag it into the Viewer window and use the default keyboard shortcut "F11"
and that will replace the clip under the playhead according to the track targeting ... tho you note these sequences are the same length anyway so why not just drag and drop (overwrite)?
If its the latter, then I think you're out of luck. I don't know of any shortcut that "auto" closes the open sequence when opening a new one, although you can speed up closing it yourself by mapping the "Close Other Tabs" shortcut to something suitable for you ... I tend to just open as many sequences as I need into separate tabs and then toggle between them... use the "Ctrl W" shortcut to close the active tab, or "Close Other Tabs" if I want to concentrate solely on the active one.
Hope it helps
Andy