The symbols are cached with the design so that if the library doesn't exist or the path cannot be resolved you will not lose any data. You can continue to add copies of existing symbols but not add new symbols unless you point to a library. The Setup- Settings - Symbol Libraries is only a pointer to where the libraries are stored on disk, so changing this setting in the dialog will not move the real files from location X to location Y, this probably accounts for the error message. If you move the library folders from X to Y and then change the pointer in the settings dialog you should not get this error. Also, provided you keep the symbol library folder structure the same (same symbols in the same library just moved from C: to E: say) then there should be no issues. DxDesigner does not like symbols being moved between libraries.
This only applies to DxDesigner symbol libraries, I cannot answer for the PADS PCB library changes you mention.
Thank you very much! I got it! Actually I have done everything right but the error still was present. Then I thought that the source address may be too long, as I have used many-many folders:
E:\Projects\SSMD\CAD\Design files\SSMD_Rev1-2_[03.08.12]\DxDesigner Schematic Diagram (Rev1-2)\Element Library
So I have moved the Library folder to another location (not so far away from disk E:\ and now no error appears.. May be it did not like name of some of the folder... I do not know. But anyway, now everything is ok.
Could be the length, but my guess is the use of special characters and spaces in your folder names. I see square brackets, parentheses, dots, dashes and spaces. We (Customer Support) recommend a limited character set, from TechNote mg518308: DxTopics: Best Practices for Using DxDesigner http://supportnet.mentor.com/reference/technotes/public/technote.cfm?id=mg518308:
Avoid problems with names including folder and project names
The recommended character set to use for name is A-Z, a-z, 0-9 and _ (underscore), this set will minimize the chances of encountering problems. These characters can be used consistently for all names with no known problems. Using other characters increases the risk of encountering a problem. One example is the space character, which occasionally will introduce unpredictable problems.