Hey Mark,
Here is how you could accomplish that:
1. Go to Window > Preferences > MyEclipse > Project Capabilities > Struts > Struts 1.2
2. Remove all jars and tlds
3. Download and unzip struts-1.3.5-all.zip distribution (full distro contains TLD files)
4. Under the required libraries do “Add Jar/Zip”, navigate to your struts-1.3.5/lib directory, select all jars, choose open
5. Now for the TLDs… you will need to add each tld that is available in the struts-1.3.5/src/*/src/main/resources/META-INF/tld/* directories
Once you have added all the updated jars and tlds choose OK. Now all of your struts projects that are configured with Struts 1.2, will now be struts 1.3.5 projects. Hope this helps.