Powershell - I Need to move CustomAttribute10 to CustomAttribute15

Hello;

I am still cutting my teeth on this PowerShell stuff and need assistance. I thought pulling a Mailbox Attribute value into a variable then writing the variable data back would do the trick, but I end up with this..

Before:

CustomAttribute10 = "Sparky10"

CustomAttribute15 = NULL

After: (Bad Result)

CustomAttribute10 = "Sparky10"

CustomAttribute15 = "@<CustomAttribute10=Sparky10"

I NEED: (If someone can assist)

CustomAttribute10 = NULL

CustomAttribute15 = "Sparky10"

The value from CustomAttribute10 will need to be essentially copied to CustomAttribute15, then CustomAttribute10 will need to be cleared. I used Sparky10 as a test attribute value, but every user's actual CustomAttribute10 value will be unique.

I will be doing this for every mailbox on our system which is over 100,000 of them. If the size of the query is too big I will need to deal with that too. 

Thanks in advance.