Hi,
I have a website (portogente.com.br) and we're currently running mijosef, we are very unhappy with then and we're now uninstalling from our Joomla installation, the problem is that our URL, more than 100k are all in the format like /item-alias-(item id) and the k2 routing is /(item id)-item-alias, how can i swicth this so the item ID is at last in the URL?

You can try with K2's advanced SEF but I do not think it will provide the solution you need.
Unfortunately you will need an SEO extension like sh404sef or use a pattern for mass 301 redirects to the new URL pattern.