Your question is based on a premise that is flawed: Satan was not based on Pan. The idea of Satan, an evil, near-deity, has its strongest roots in Persian Cosmology. Christians brought the concept with them, drawing on their Judaic roots, which were influenced by religious traditions to the east. As Christianity spread into Europe, it encountered indigenous traditions - including poor innocent Pan, who was not evil. Nevertheless, some of Pan's attributes bled into that of Satan. It does not follow that people were worshiping Satan before they became Christians.

European folklore often conceives of an earlier period when giants dominated the landscape. This does not refer to an earlier belief system, but rather, simply, a projection into a mythologized past. As we peel the onion to its core, we will not find a time when people worshiped giants. In fact, the onion metaphor probably fails us altogether. Rather, it is a matter of an evolving belief system that evolved over time, absorbing and transmuting foreign influences, but the idea that there was an earliest layer when people worshiped titans and then threw them off for a later pantheon does not fit what can be observed.