Yes, you can copy the saved data log files (including the trend data) from one project to another. Using Windows Explorer or similar program, look in your RSView Project file for a file folder called "DLGLOG", which is the saved DBF-type log files for all your RSView historical data that has been saved. Pick the dates (hopefully your files are saved using a date in the name) you want to copy, and copy and paste them to the DLGLOG folder in your new project.
To view the old trend, you will need a trend chart set up identical to the original trend (same tags and same number of pens, and so on).