I Instrument an app old build in my system and install latest build in the device. When i execute the script it's working fine. My approach is it will always work or in some type of scenario will create some problem.
Not sure I got your scenario... Instrumentation is required in order TestComplete to get access to tested application internals and to make it possible for TestComplete to search for controls, access their properties, etc. With the application not been instrumented, your test code that tries to access application's internals will fail.
On the other hand, you can use image-based testing and identify target controls in the tested application by their images. This approach does not require instrumentation but obviously is slower (in terms of execution speed) and less reliable.
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Your approach will work until something in the latest build breaks your test or there is a feature in the latest build that your test doesn't cover. A broken test will be obvious but you may miss important testing by not using the latest build.