If you are able to use the Trading API call ReviseFixedPriceItem then you could change from "Item Weight" to "Weight" using ModifyNameList. You need the item numbers.
Here's what the XML looks like for one listing revision:

<?xml version="1.0" encoding="utf-8"?>
<ReviseFixedPriceItemRequest xmlns="urn:ebay:apis:eBLBaseComponents">
<ErrorLanguage>
en_GB
</ErrorLanguage>
<WarningLevel>
High
</WarningLevel>
<Item>
<ItemID>
123456123456
</ItemID>
<Variations>
<ModifyNameList>
<ModifyName>
<Name>
Item Weight
</Name>
<NewName>
Weight
</NewName>
</ModifyName>
</ModifyNameList>
</Variations>
</Item>
</ReviseFixedPriceItemRequest>

For info see:
https://developer.ebay.com/devzone/XML/docs/Reference/eBay/ReviseFixedPriceItem.html#Request.Item.Va...
https://developer.ebay.com/api-docs/user-guides/static/trading-user-guide/update-variations.html

You could also change variation values but this has to be done by deleting the affected variation(s) first and then adding new ones.