I have a set of data charted and a trend line for the data (in Excel). Is there a way to change the slope of the trend line and it will adjust the data to match the new slope?
No. However, you can take the equation for the trend line and calculate the data that comprise it, then create a new data set that adjusts for the slope of the trend line that you want to use.
To get the equation - slope and offset - of the original trend line, select the trend line on the chart, right-click and choose Format Trendline. At the bottom of the dialog box that opens up, there is a check box for Display equation on chart. The equation will be put in a new text box on the chart, and you can copy the equation directly from the box.
Prior to Excel 2007, it was possible - by changing the position of a point on a chart - to change the underlying data as well. In response to user feedback, a free addin was developed to provide this kind of capability. I have not tried it, but perhaps you will find it helpful.