Transfer of budget in H2020 is allowed between cost categories without requesting amendment. However, is it required to communicate about these changes to the EC, for example in the periodic report?

In the periodic report you have to report the work carried out – not what you had planned to do. So yes, you have to at least communicate it within the periodic report. Also it never hurts to let your project officer know what you are doing.Highsnobiety Sneakers