It's cumbersome, but the method below (from VW tech support) worked for me:
When you open the 2017 file in 2018, click on the Title Block Border (Formerly Sheet Border), go to Object info palette (OIP) and click Update. That’s where you’ll probably see your Title Block Border shifted. In the OIP, click on the Style dropdown and choose Convert To Unstyled. Then click on Title Block Border Settings. In the Title Block Border Settings dialog, click on the Sheet Border option. Change all of the margins to 0. Then click OK. Title Block Border should then go back to it’s original position. You’ll want to save this as a Style to be able to use this on all of your sheets. To do this, go to OIP and choose New Plug-In Style From Unstyled Plug-In. You will then be able to use your custom Title Block Border as a style to be applied to all of your sheets.
In order to apply the Title Block Border Style to multiple sheets, you’ll need to have a Title Block Border already applied on your sheets. If you haven’t updated the Title Block Borders you have on all of your sheets, make sure to do that first. Then go to Tools > Custom Modification. Set the criteria to be “Type” Is “Title Block Border”. Once you click OK, this will select all of the Title Block Borders in your file. In the Properties dialog, choose your Title Block Border Style and click OK to have it applied to all sheets.