If a church does not believe that the bread and wine become the Body and Blood of Christ, does this mean they don't actually become the Body and Blood of Christ (the real question here is why are the bread and wine transformed in Orthodox churches?)
The answer to the first part of the question is YES. In his book, "The Seven Sacraments", HG Bishop Matthaos sets out the three conditions for a sacrament to take place validly:
1. An adequate substance for the Sacrament like water for Baptism, bread and wine for the Sacrament of Communion, oil for the Unction of the Sick, and so on.
2. An ordained priest by the laying on of the apostolic hand.
3. Invocation of the Holy Spirit by the priest, by praying certain prayers for the dwelling of the Holy Spirit and sanctification of the Sacrament.
A church that does not believe that the bread and wine really become Bidy and Blood is almost certainly a church that also does not believe in the sacrament of priesthood either, and does not have an Apostolic origin where the gift of the Holy Spirit that underlies the priesthood is passed down from one generation to the next. Neither would they ask the Holy Spirit to chnage the bread and wine. Thus, it would certainly not be changed.
Regarding the second part of your question: The basis for the Orthodox beliefs about Communion is the Bible and the Traditional interpretation of the early Church. For example:
“Take eat, this is My Body. Then He took the cup, and when He had given thanks He gave it to them, and they all drank from it and He said to them “This is My Blood of the new covenant which is shed for many” (Mark 14: 22-24).
“Whoever eats My Flesh and drinks My Blood has eternal life and I will raise him up at the last day. He who eats this Bread will live forever” (John 6:54,58).
“For My Flesh is food indeed and My Blood is drink indeed .... As the living Father sent Me, and I live because of the Father, so he who feeds on Me will live because of Me” (John 6:55,57).
“For we being many, are one bread and one body, for we all partake of that one body” (1 Corinthians 10:17).
Our Eucharist follows the pattern of the Last Supper, where Jesus Himself consecrated (or blessed, sanctified) the bread and wine and said quite clearly and bluntly: "This is My Body ... This is My Blood."
Further, we find St Paul warning the Corinthians for not respecting, even for not recognising that they are in the presence of the true Body and Blood. He goes so far as to say to them:
27Therefore whoever eats this bread or drinks this cup of the Lord in an unworthy manner will be guilty of the body and blood of the Lord. 28But let a man examine himself, and so let him eat of the bread and drink of the cup. 29For he who eats and drinks in an unworthy manner eats and drinks judgment to himself, not discerning the Lord's body. 30For this reason many are weak and sick among you, and many sleep. 31For if we would judge ourselves, we would not be judged. (1 Corinthians 11)
If this were not truly the actual Body and Blood of Christ, if it were only a symbol, would people fall sick because they did respect it. or even die for that reason?
This Q&A was found on Archangel Michael & St Bishoy Coptic Orthodox Church : http://www.stbishoy.org.au/modules/smartfaq/faq.php?faqid=82