Well, finally, a kind co-worker gave me a trick to solve this problem.

May be there are righter ways to manage what I want, but this works perfectly:

I left my create button, which is inside a footer. What I did is to add a attrs tag to this footer, I made it invisible if the ID of the record is different from False. If I'm creating a record, it doesn't have an ID yet, but if I'm reading, it does. Obviously I also had to add the field ID to the fields shown in the form, otherwise it couldn't be used in the attrs tag.

<footer attrs="{'invisible': [('id','!=',False)]}">
    <button name="create_rplc" string="Create" type="object"
        class="oe_highlight" />
    or
    <button string="Discard" class="oe_link" special="cancel" />
</footer>