The difference between 39 and 34 in front is 13% required effort per pedal crank revolution. To get the same difference only using the rear you would have to change from a 25 to a 29 largest cog (if it existed). If you do both, go from 39/25 lowest gear to 34/28, you can get a total of 23% reduction. Assuming you want to stay with a road rear dearailleur and don't want or have Apex, that is the best you can do. That is huge. The good news is that is doesn't much hurt your top end. Not too long ago, 12t was the smallest rear cog. Now with 11t cogs, you can have a very slightly higher top gear with 50/11 than you used to have with 53/12. What does change a lot is the spacing of the gears and the changeover point that you have to go from small front ring to large and vice-versa. That is a new learning process. You may not like the arrangement of gears in a compact setup, but it is the price you pay for easier climbing. One way to analyze things is to take note of which cruising gears you use most now and see how close you can get to them (not the same combinations, but just the same ratios) with the compact gearing. Don't forget to take into account any new rear cassette you would opt for. Can you cruise at the same exertion or not? Are your three favorite cruising gears reasonably situated to each other to make shifts convenient. That is what you really need to determine. For example, say you like 53/17. That is a ratio of 3.12. With a 50 front, that corresponds to a 16 rear nearly perfectly. Not too bad if you have it or can get it on a new cassette. And so on...