This was a post I created and augmented several times in the last few months:
Summary: repair hammers become material specific, both retaining their ability to repair damage, but offering the ability to change the material.
Application: A single repair hammer exists to allow multi tier buildings to be repaired without material change, the remainder cease to exist and are replaced by 1 hammer per building material. Sandstone, stonebrick, insulated wood, reinforced, and black ice, and dlc specific hammers.
Function: if targeting a damaged block with its matching hammer, the hammers will function identically to current repair hammers. If the player had the required materials, each swing will add them to the block and repair its durability.
Alternatively, if the player targets a block with a hammer that does not match its material type, the same screen repair pop-up is shown, this time offering the total building cost of that block in the hammers material. If the material is on hand, each swing will fill the requirement and upon completion the block (and what ever blocks being hit by the hammers AOE, as it functions currently.) will begin to change into the new material.
Benefits: often in natural player progression upgrading your current base to newly obtained tiers is a hassle, and many times you cannot reach blocks buried beneith others. The current repair hammer coding allows this to be a thing of the past.
Also! My favourite part is that building a base often comes with mistakes that you must take down, thus wasting your hard earned materials, building in sandstone is quick and cheap, and upgrading your final product would save a lot of valued resources, and allow players to experiment more before finalizing a plan.
Obviously retaining the ability to build blocks in menu would be positive for players who want to quickly replace destroyed blocks, or Create custom structures from multiple materials.