I am upgrading Splunk from 6.5 to 7.3, Do i need to upgrade event collector agents as well, Or the existing agents will be compatible with the latest version.
There is no reason that you must but some effort at some point should be made to upgrade the UF tier. The farther away the versions get, the more complicated the upgrade will be and the more "in-between" stops on intermediary versions it will take. To go from v6.* to 8.* UFs should be a simple upgrade UNLESS you are using SSL/TLS; there is some upgrade breakage there but the docs are pretty clear about it.
Without know the details about which Uber agent version is installed, if it is standalone or on a UF, best i can do is point to this article:
https://uberagent.com/docs/uberagent/latest/about-uberagent/system-requirements/
Also if it is installed along with UF, review this:
https://docs.splunk.com/Documentation/Forwarder/8.0.2/Forwarder/Compatibilitybetweenforwardersandind...