Being a Pro subscriber means that you get access to all of our Premium icons under the Basic License. This means that, you can use the icons for commercial purposes, without giving attribution to the author of the icons.
You can use the downloaded icons forever, as long as you abide for the terms from the License agreement.
By signing up for Iconfinder Pro, you have the right to use all icons under the same license agreement: the Basic License. This means that, as a subscriber, you don't need to worry about different license agreements for each icon you download, but same terms apply to all icons.
This does not mean that the license of the icons changes. The original license stays the same, but you get the right to use the icons under the Basic License.
Once you stop being a subscriber, you will need to pay attention to the license agreement of each icon.