Thanks Prema for reading.
To your first question – Can one zoom in and out once focus is locked? You can zoom in and out, but the question is whether the subject will still be in focus. Depends on the lens. As per specifications most of Canon L series lenses are supposed to. But I will never trust that. If I zoom in or out, I will focus again.
Do you also change to Spot Metering? First spot metering should be used with care. Unless I have a reason to use Spot Metering, like back-lit subjects, I will not use spot metering. Since we will be recomposing after acquiring focus, there is no point in metering just on that point. The answer is NO.