The Codex Astartes is the sacred text of all Space Marines.
The most important rule to remember while reading it, is that “the gene-seed of a Space Marine Chapter cannot be used for any other purpose than creating more battle brothers.”
This means that once you have an army of marines, there’s no way to convert them into anything else. The Codex Astartes does not support this action, but it is possible to convert an army of marines into a force that closely mimics the function of Space Marines.
The only way for a player to accomplish this conversion would be through careful and meticulous planning.
No matter how you do so, converting your marine army will always take time and effort on behalf of both players during game play.
An example might be as follows: if I had 20 infantry units in my squad with nothing else available then I could use those squads as Tactical Squads by removing their bolters from them at the start of every fight phase and replacing them with storm shields making sure they never shoot anything until turn three where they can charge