My dad died three years ago, he was a very respected elder in the congregation and although my family is in two halves (one side now 1.5 people witnesses the rest not) we all attended the cremation. It might sound incredibly stupid or naive but i went with a completely different hairstyle (i'm naturally curly but went straight) and didn't wear anything that I would have worn back in the day. It worked for a lot of the people there, especially the busy bodies who would have cornered and surrounded me - by the time they worked out who I was i'd scarpered and was talking to other people or had disappeared.
It didn't fool everyone though but it made me feel better knowing that i'd changed so much since i left and nothing any of them could say wanted me to go back, or even talk to them outside of the crematorium. I did get the obvious comments (and one person really p'd me off and i wanted to swing for him) but i smiled and said thank you for coming or you're looking well or thank you for your condolences and walked away. No scene, no members of the family glaring at me for being rude.
Of course, all situations are different but if you're calm and polite (despite what you'd like to say them or tell them where to go) and try and keep away from the people you know will give you a hard time it'll be ok.
Hope all goes well and remember, it may have been so long they'll hardly recognise you.