In general, as a rule of thumb,
Any modifications minor or major to both external and internal areas of the watch, made during the effective warranty period will in effect void the warranty.
Do take a look at the warranty terms & conditions printed in the warranty booklet, the above phrase can be found somewhere in there in different variations but same meaning.
However, some watch brands do offer in-house customization services (eg. engraving) at their AD and service center, if its done by them, then the warranty usually is still effective.
