Migrating applications with different Appian versions

Yes this is correct. You're able to promote Appian objects from a lower version of Appian to a higher version but not vice versa. Although with every Appian release, objects may be deprecated. You can verify that by going threw the release notes and Appian documentation.  Please refer to https://docs.appian.com/suite/help/17.3/Application_Deployment_Guidelines.html

I agree with Jamal. Just want to highlight, one of the major deprecated feature is http query smart service and function which is replaced by a Integration rule. Even we are upgrading from 17.1 to 17.3 next week, and it shouldnt be a problem.

One minor disruption will be needing to make manual updates in some areas. For example, the places where you called a!formLayout will appear like this after upgrading to a higher version:

 

> One minor disruption will be needing to make manual updates in some areas

I'm not sure I'd even qualify this as a minor disruption, since no change will be required to individual forms until they (individually) need to be updated to utilize the newer functionality.

Hi jarrods229 Also you need to think about removal of function or component or any feature (if any) while migrating from lower version of Appian to higher version.

Because when something is planned to be remove in future, they flag that as deprecated and in next few versions they remove that. So make sure, you are not using any one of those features which has been removed in current or later versions of Appian.

Such as "Attachments and Notes on Process Models" which was removed in Appian 17.2 , so you need to cross check these as well while migrating from 17.1 to 17.3

we can deploy application from lower appian version to higher appian version, but before that please analyze that whether appian components used by application are deprecated in higher version or not.

You may already use Health Check. In case you don't, Appian Health Check is a useful tool and tangentially related to your question for understanding potential risks in your environment/application. More info can be found here: community.appian.com/.../appian-health-check