You will need some sort of script to manipulate and transform the data from one form to another. You could do this using some C# code and utilising the Sitecore API but my personal preference would be to use Sitecore PowerShell Extensions as you are planning to do.
In order to "migrate" the data from the old 7.2 instance to the new 8.2 instance you can use any of the options suggested by @Diego in his answer or use the Express Migration Tool (and select to migrate Items only).
This will then allow you to more easily work with the data in your SPE scripts since it is then a "local" instance and you do not need to mess around with the Item Web API as well as having the advantage of providing a richer API in my opinion. You can treat the items as data only and write transform scripts to create new items based off the old data in whatever structure you need, then finally delete the items once you have finished.
Secondary Master Database
If you really do not want to bring the old data into your new instance and want to treat it as a separate data repository then I would consider attaching the old master database from the 7.2 instance to your new 8.2 instance and then working with it directly from your SPE scripts.
- Attach the 7.2 instance database to your SQL server and add a connection string entry:
<add name="legacymaster" connectionString="Initial Catalog=sitecore72_master;..." />
- Copy the
sitecore/databases/database[@id='master'] node and rename the
id attribute to match the connString:
<database id="legacymaster" singleInstance="true" type="Sitecore.Data.DefaultDatabase, Sitecore.Kernel">
- You can now reference the old databse form your 7.2 instance directly in your PowerShell scripts to access the data and then copy/transform/create items in the master database of your new 8.2 instance:
# Get the original item from the 7.2 instance
$olditem = Get-Item -Path "legacymaster:\content\home\oldpath"
# Create a new item in the new 8.2 instance
$newitem = New-Item -Path "master:\content\home\sample item\Sample Item 3" -ItemType "Sample/Sample Item"
# Edit the item to set correct values of fields etc.
$newitem["New Field"] = $olditem.SomeField
$newitem["Other Field"] = $olditem.SomeOtherField
#add any other changes you need
Once your migration and transformation is complete you can delete the old database, connection string and config patch. This should providing an easier mechanism for you to write your scripts rather than using the Item Web API in my opinion.