What to do if a manufacturer will not honor “lifetime” warranty?

You could sue the manufacturer for breach of contract and specifically breach of express warranty.   As a "lifetime" warranty it is not limited to a year. In addition to a cause of action (claim) in your lawsuit for breach of express warranty, you may have a claim for breach of implied warranty of merchantability.  The implied warranty of merchantability means that the fan has to be of a quality acceptable in the trade.    Another type of implied warranty is the implied warranty of fitness for a particular purpose.  Since you did use the fan for a year, it may be difficult to claim that there was a breach of the implied warranty of fitness for a particular purpose if the fan was functioning properly.

It is also possible that you may have a claim for fraud since the company's "lifetime" warranty is only honored for a year.  Fraud is the intentional misrepresentation of a material fact made with knowledge of its falsity and with the intent to induce reliance on which you justifiably relied to your detriment.  Your argument supporting a claim of fraud is that you paid for a lifetime warranty which the company intentionally misrepresented to you and is now claiming that it is only a one year warranty.  You justifiably relied on a lifetime warranty to your detriment by paying the additional charge for that warranty.