mancini, I am pretty much sure. As long as your previous employer holds your H1b there shouldn't be any problem. If your previous employer had fired you or you got laid off and you are trying a new company with H1b transfer which got denied then no one is holding your H1b. In that case you will be out of status.
In fact, You can have multiple H1bs at the same time if you work for multiple employer at the same time.
I suggest sajha members to google it before you make any decisions. There are tons of people who have been through your situations.
For example here is one example
http://immigrationvoice.org/forum/forum108-analysis-discussion/1127153-what-happens-when-h1b-transfer-gets-denied.html
macini, your case is different. You are NOT in h1b status right now. Your official H1b status will start on October 1st only. In your case if your new company wants to file new H1b before October 1st there should be enough quota available(which is already over).
So my best suggestion for you is ask your new employer to wait till october 1st and file the new LCA after that(AKA h1b transfer). In that way your new company won't need to have quota available.
I know this is very confusing and it took me few years to understand this whole mess.