Remember that the pH scale is logarithmic. pH 6 is ten times more acid than pH 7, but pH 5 is ten times more acid than pH 6 (and a hundred times more acid than pH 7). Orientals will do fine even at pH 5. In the end, you will never make your soil too acid. But it is certainly possible to do it temporarily with chemical based products, like iron sulfate, aluminum sulfate, ammonium sulfate, and even just sulfur.
The peat moss is semi permanent. If you mix a cubic foot of soil half and half with peat, with your native soil around it, I think you would need to redo it every 4 to 5 years. If you amended a bed (say 6x6ft) half and half, then I think more like 10-15 years. Adding the aforementioned products, or sulfur in small quantities can only help, but it's easy to over do it. Apply only once a year, sprinkle as you would a fertilizer (best to read the directions, of course!). Don't every use powdered anything, only granules or flakes. Powder is too fast acting and the soil pH will take a rollercoaster ride. Sulfur and sulfates work by "turning into" sulfuric acid to acidify the soil. Unfortunately sulfuric acid is very mobile in the soil and easily drains away with rain and watering. Peat makes soil acid by ion exchange (put simply) and is generally not susceptible draining out of the soil.
If you have already growing lilies in pots, or if you want to buy already growing lilies or if you already have bulbs, sure, go ahead and plant now, realizing that you're planting for next season, and this season they will look pretty ugly. it will be better than waiting until fall to plant them. If you want to buy more bulbs, buy them for planting in the fall.