Conversion of old equipment to variants, i.e pzI to panzerjäger I (tank destroyer) will come later after launch. Scraping of old equipment wont be implemented tho. Historically that didnt really happen on a large scope and if anything was scrapped it were ships of which the construction got cancled after work already started (German Carrier Graf Zeppelin and the H39 Battleships for example).
You can however make use of outdated equipment by lend-leasing it away to allies with a weak industrial base to get some land/air experience in return to upgrade your newer tanks and planes. You can also use them to train new divisions of course.