This weekend a customer of mine is having a major system change which results in new ISP's, Servers, workstations.. the whole shebang!

My role is to
a) Reconfigure their existing Draytek routers (a variety of 2800's, 2820's and one 2600)
b) Change the IP ranges to fit in with what the new system vendor requires.

Current setup is as follows
Site A = 192.168.6.x (router IP is 192.168.6.50)
Site B = 192.168.67.x (router IP is 192.168.67.99)
Site C = 192.168.1.x (router IP is 192.168.1.99)
Site D = 192.168.5.x (router IP is 192.168.5.99)
Site E = 192.168.4.x (router IP is 192.168.4.99)

Desired Setup
Site A = 192.168.10.x (router IP is 192.168.10.1)
Site B = 192.168.20.x (router IP is 192.168.20.1)
Site C = 192.168.30.x (router IP is 192.168.30.1)
Site D = 192.168.40.x (router IP is 192.168.40.1)
Site E = 192.168.50.x (router IP is 192.168.50.1)

VPN's currently exist between all the sites. VPN's will be recreated once they're all been migrated to the new ISP.

There is a SCO server at each site. None of them are under my direct control and I don't have root access to change the IP addresses. These servers will eventually be made redundant but for a short period there may be occasions where staff require access to them to review past transactions.

I had a SpeedTouch 780WL router provided to me a few years ago when I was with BeThere. This router had the ability to have multiple LAN IP addresses. eg, it would by default have an LAN IP address of 192.168.1.254. I added a LAN IP address of 192.168.150.1 to it. My DHCP server handed out IP address in the range of 192.168.150.100-200 and were able to connect to the internet. If I was to set a PC statically to have an IP of 192.168.1.10 and a default gateway of 192.168.1.254, it would also be able to access the internet.

What I'm wondering is, is there a way of giving a Draytek router two LAN IP addresses so that users can continue to access the SCO servers on the OLD IP range.

Please note : I know that PC's on the SAME LAN as a particular SCO server don't require this. I can simply add a 2nd IP address to the PC which is in the OLD IP range (by going Local Area Connections Properties -> Internet Protocol Version 4 (TCP/IP) -> Advanced -> IP Addresses -> Add). I'm hoping to allow users to access theses SCO servers over VPNs (which I'd need to setup ;))

Apologies for going into a lot of detail but the crux of the matter is - Can I give a Draytek router two LAN IP addresses and if so, how?

I take that as a no then?

Right, NO.
(none of the models you have are able to have more then 1 Lan Address)

I think it should be possible by making improper use of the 2nd subnet 'ip routing' feature from the LAN - General Setup section, give the router a 2nd subnet IP on the old subnet and that and the client PCs should be able to access that SCO server, the downside is the SCO servers won't be able to access the internet but your description implies that won't be necessary so I think it should work as a temporary solution.