Hopefully this is just bits/bytes confusion, and you're seeing 100 megabyte/second downloads and all is well? Or are you looking to do better than gigE?
It's pretty unusual to actually have cat5 and not cat5e or at least cable marked as cat5 but substantially exceeding the specs.
Only the most recent 'multigig 2.5/5g/10g' ethernet devices actually do anything approaching measuring the quality of your lines. Standards compliant 1g and 10g equipment will be happy to negotiate to the highest shared standard over a 2-pair cable through the 1mbps link pulse data and then be unable to communicate ethernet frames if pairs are missing or the wiring isn't sufficient quality for the encoding. Although some drivers compensate and reduce the advertised capabilities when there is no data exchanged after negotiation completes.
It usually takes active steps to negotiate down to 100M with gigE equipment.
All that said, 100M/100M is still a pretty solid experience if that's what you can get.
As for getting people on board, you'd need to get an idea of the cost, and then probably propose it to the HOA board and/or at a meeting. Expect most people not to really care though. 100M gets you 5x solid video streams (netflix and/or video conferencing), and in a dense condo building, wifi speeds are going to be less than that due to spectrum congestion anyway.