I’d like to change my current chainring combination (ultegra 52/36) for something smaller. I’ve two options at home, a 46/36 and also a 49/36. The 49/36 is my preference but I’m not sure if the 49 is compatible with the 36. Any advice would be welcome and would save a whole lot of tinkering about 
I wouldn’t want to lose the top speed of my 52 x 11 but for a trip to Tenerife which is a bit hillier (more mountainous) than here I swapped my inside 36t ring for a 34T. I had wanted to do it very cheaply and fit an Ultegra ring I had on my Dura Ace chainset but it was incompatible and I had to buy a Dura Ace ring.
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What brand is the 49? 
I’d just find a used 50/34. Otherwise, could you source a 34t to use with the 46?
46/36 is a bit redundant in my opinion and was only ever common in CX if I’m not mistaken. It will work for sure, but you’re not really getting an easier gear if that’s what you’re after. In that case, the 52/36 arguably makes more sense.
Also, what type of bike is it?
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I think this is the answer. Making some assumptions but if OP is on an Ultegra 52/36 then there’s plenty of 50/34 24mm spindle options from Shimano and otherwise without putting a new BB in the bike. Additionally, if for their use OP feels going smaller is needed, or for a trip to big mountains they’d have the GRX cranks available which offer a wide ratio, smaller gears and avoid the tight 46/36.
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Yeah the GRX stuff came to mind (I like the 46/30 with an 11-30 cassette) but it depends on the type of bike they have and if they have a GRX FD which is required for those cranks.
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I’m not sure what the official Shimano line is, but I’ve used the GRX cranks in 48/31 with an Ultegra 11spd Di2 front mech without issue.
If you want smaller gears, I’d assume you want them when already in the small ring. (Otherwise, just shift down earlier!) Which means, swapping the 52/36 for something else with a 36t small ring is not going to help you. The next common size down is a 34t inner chainring. However, the 34t inner is not recommended with a 52t outer, or shifting becomes bad. So you’d have to swap that too. I’ve never seen a 49t chainring, but a 50t is the most common combination with a 34t inner chainring (also called a ‘compact’ chainset). Think it’s probably the most frequent combination now, so should be easy to find.