I have selected another office as ISA. Is it possible to change and select the EPO as ISA instead?

No. Once you have filed your application and selected another ISA than the EPO, it is no longer possible to change.

However, you have in any case the option to request the EPO to perform a supplementary international search (SIS). A SIS request must be filed with the International Bureau of WIPO within 22 months from the priority date. If your application was filed in another language than English, you must also file an English, French or German translation of it with your request. The EPO will establish the Supplementary International Search Report (SISR) within 28 months from the priority date.