Zafar,
There are some new services in 6.5 that would not work on a 6.1 server. Also, some built-in service may have changed slightly.
The best way to prepare for this is to review the release notes for 6.5 to identify any deprecated services, new services or changed services. Unfortunately, the release notes may not identify each of these cases.
You could probably create temporary coding standards to avoid most issues, but should certainly make thorough QA testing on a 6.1 IS box a part of your deployment process.
Also, Developer 6.5 can connect to IS 6.5, but you will get a warning about some unsupported features.
Mark
#webMethods-Upgrade#webMethods#webMethods-General#Integration-Server-and-ESB