Hi Batata,
Welcome to the world of soundfont creation!
Even though there is no "search and replace" function for parameters in Polyphone, there is still a very quick and easy way to modify velocity layers:
1. On the top right of your screen, there should be some icons that look like this.
2. Click the arrow pointing both left and right. This should change the table of parameters to a screen which visualizes samples according to their velocity range and key range.
3. Now, shift click the lowest sample of 71-89, and then shift click the highest sample of the same velocity. All the samples of 71-89 will be selected.
4. Go back to the sample listing on the left, right click on one of the selected samples, and click delete. The space where the samples of 71-89 were should now be empty.
5. Repeat step 3, except with the samples of 49-70. All the samples of 49-70 will be selected.
6. Drag the top end of one sample up until the blank space is gone.
7. Test that you have done it properly by using the virtual keyboard, and play a note with velocity between 71 and 89. If there is no sound, make sure that you have dragged the samples of 49-70 up so there is no blank space.
8. Once you have done, click the table on the top right of the screen to return to the parameter table.
Note: Don't forget to "remove unused elements"!
I hope this helps you,
Sylvia.