Is that possible to switch an Open Source Project license from GPL to LGPL v3 ?
I am the project originator and the only contributor.
yes, of course. You can change the licence to whatever you want.
Go to your admin page, then edit registration from the menu, view Public Info, then edit the Trove categorisation. You need to remove then add a new category. Easy (if a little link-happy).
This applies if you're an admin of the project, no matter how many of you there are or who has contributed.