Replacing Live Rock in a mature tank

[majorstare]

My tank, a 2ft sumped cube, has been running in various forms for 6yrs.
Currently LPS, SPS and softies. ATI essentials, Deltec skimmer, phosphate reactor, 2ltr of Siporax.

The LR is old and its suffering from nucience algae, specifically a tough, red algae which is extremely hard to remove/trim back.
I'm also looking to have a minimalistic, low scape in 3 sections on the sand bed, rather than the existing bonsai structure.

What's the best way to replace the existing LR and put in the new scape structures with new LR?

1st Picture is current (lights dimming down for the evening).
2nd Picture is similar to what I would like to achieve.

 

[Alfrareef]

Can you remove, scrub and place old LR inside the sump?

 

[majorstare]

Can you remove, scrub and place old LR inside the sump?

Not really, sump is full.....

 

[Alfrareef]

I see some space left...
Here’s what I would do:
. Place the new LR inside a container with new salt water with a circulation pump and a heater for a few weeks until the NO3 and PO4 are ok;
. Get the old rock out and scrub it real well inside a container with tank water and then place the most of it inside the sump. You can fill the spaces that you have and it will help the tank since the new layout seems to have less LR;
. Get the new LR and place it inside the tank;
. After a few weeks you can remove some of the the LR inside the sump.
My 2cents...