Switching from an IP to an other, is one way for collecting massive data without being busted.

Anyway, if the Site Source formally denies Crawling, using such technical measures remains definitively an infringement to their user license, which can be at the end a legal issue.

Before crawling, be sure being authorized by the Site Owner, even more, before playing these kind of technics, which could easily be assimilated at the end to a denial of service attack.