You have to do this in the nat table and the prerouting or output chain, and you should use DNAT instead of REDIRECT, because REDIRECT is for packets with destination your own machine. So we have:
iptables -t nat -A PREROUTING -p tcp --dport 515 -j DNAT --to- destination remotehost.com:900
I would suggest that you add a destination match as well so that you don't blindly redirect traffic destend to any system's LPD port. I.e. make your IPTables like look like this:
iptables -t nat -A PREROUTING -p tcp -d remotehost.com --dport 515 -j DNAT --to-destination remotehost.com:900
Grant. . . .
