In early mission phases PIs were allowed some latitude with observing strategy due to uncertainties about instrument capabilities. While some changes are still permitted, these are far more limited. All changes must be consistent with the original aims of your proposal, as approved by HOTAC.
If initial data shows that your targets are fainter than expected, you may, for example, reduce the number of targets to allow you to go deeper on each one or, if you request both PACS and SPIRE photometry, you may request that no more observations be taken with one instrument and all the saved time be used to increase exposures with the other.
Changes of instrument or observing mode are generally not permitted.
Note that all changes of target, observing mode, or strategy must be approved by the Project Scientist.