I am about to mount a new database, but I cant put the old server down as it is used in production.

The other one is a new version of it (Oodo a.k.a openerp). I need to build it, transfer data, get it customized properly then, when it will be working 100% correctly, save an image of the old server just in case or even, put it on the temporary linode and, copy the new one (on the temporary server) to the production linode (which has already been paid for 2 years.

Is it something possible to do ?

Yes. Back when wildfires in California threatened power supplies to the Bay Area (Fremont), I created a Linode in Dallas that was twice the size of my Linode in Fremont. I left enough blank space on the new server for the copied image from existing server, then created an OS on the new server. I also attached it to a block device so I could mount the copied image of the original server.

You might need to ensure that the configured IPv4 addresses on the images work with the Linode it is running on.