1) In addons\product\product.py File

Temporary uncomment the line and then restart the server after that change UOM

def write(self, cr, uid, ids, vals, context=None):
        if 'uom_po_id' in vals:
            new_uom = self.pool.get('product.uom').browse(cr, uid, vals['uom_po_id'], context=context)
            for product in self.browse(cr, uid, ids, context=context):
                old_uom = product.uom_po_id
                if old_uom.category_id.id != new_uom.category_id.id:
                    raise osv.except_osv(_('Unit of Measure categories Mismatch!'), _("New Unit of Measure '%s' must belong to same Unit of Measure category '%s' as of old Unit of Measure '%s'. If you need to change the unit of measure, you may deactivate this product from the 'Procurements' tab and create a new one.") % (new_uom.name, old_uom.category_id.name, old_uom.name,))
        return super(product_template, self).write(cr, uid, ids, vals, context=context)

OR

2) Change the Product UOM in database using SQL Update query

But it may be affect the functionality for the old product sale order done with uom units.