I would agree with the others, you will not gain anything by holding a ticket and if you're flying at short notice it might actually be simpler to buy a new ticket rather than spend time trying to change an existing one (that is a bit of supposition, though).

There is no problem buying tickets at short notice, e.g. It's 11:15 here right now and ba.com is offering to sell me a flight on a 12:30 departure to Europe. Considering that conformance is at 35 minutes then I think anything up to an hour before departure should be easy (although you might not get a meal loaded!)

However, one issue to look out for if you're travelling for work; you might need to get trip approvals, etc. so if this is the case you're much better off booking the ticket in advance and changing on the day.